metals technology, 1934, vol. i - december 1934 - t.p. …library.aimehq.org/library/books/metals...

AMERICAN INSTITUTE O F MINING AND METALLURGICAL ENGINEERS

Technical Publication No. 686 (CLAW C. IRON AND STEEL DNISION. NO. 118)

DISCUSSION O F THIS PAPER IS INVITED. I t should preferably be presented by the,con- tributor rn person a t the New York Meeting Februa 1935 when an abatract of the paper wlll be read. If thia is impoasible, discussion in w r i h g may?; aent'to the Secretary. American Institute of Mining and Metallurgical Engineers, 29 West 39th Street, New York N. Y. Unless special arrange- ment is made, discussion of this paper will cloae April 1. 1935. Any diicussion offered thereafter should preferably be in the form of a new paper.

Hardening andj Tempering of Steels Containing Carbides of Low Solubility, Especially Vanadium Steels

BY E. HOUDREMONT,* M E ~ E R A.I.M.E.; H. BENNEK AND H. SCHRADER

(New York Meeting, February, 1935)

THE different influences exerted by the various alloying elements in iron and iron-carbon alloys give rise to a great number of complexities, which are difficult to grasp. It is important therefore to study any systematic relations that may exist between the effects exerted by the various alloying elements, in order to obtain a general classification and thus a clearer conception of the nature of alloy steels.

A systematic survey of the alloys of iron, based on an investigation of the binary equilibrium diagrams, was recently published by Wever. Although his work deals little with properties of importance in works practice, its purpose being rather to point out the theoretical agreement among the types of binary equilibrium diagrams and the position of the alloying elements in the periodic system (atoinic radii, etc.), the classification proposed nevertheless also suggests relationships among the properties of the alloys in question, even though the alloys may contain carbon in addition to iron and the alloying element.

Wever divided the alloys of iron into one group exhibiting an open or enlarged gamma field and another exhibiting a closed or restricted gamma field, with both groups capable of further subdivision depending upon whether a continuous series of solid solutions is formed, or, owing to the separation of compounds, etc., only a limited series.

Thus all alloying elements which with iron form a closed gamma field, a "gamma loop," produce alloys that beyond the composition corresponding to the greatest extent of the gamma loop are composed of ferrite alone a t all temperatures. Alloys of this composition accordingly are not susceptible to ordinary heat treatment because of the lack of transformations. Steels of this class may thus be divided into steels that contain less than the critical amount of alloying element, and are subject --

Manuscript received a t the office of the Institute March 19, 1934. Printed without authors' corrections. Subject to revision.

* Assistant Manager, Krupp Steel Works, Essen, Germany. 1

Copyright, 1934. by the American Institute of Mining and Metallurgical Engineers, Inc. Printed in U. S. A.

December, 1934

2 HARDENING AND TEMPERING OF STEELS CONTAINING CARBIDES

to heat treatment, and those that contain more than the critical amount of alloying element and are not subject to heat treatment. Thus chromium steels are now classified as pearlitic, semiferritic, or ferritic,' a classification that can be adopted for all elements that have this type of^ equilibrium diagram. These terms have already become associated with definite conceptions in regard to properties, production methods, e t c2

Much the same is true of elements that enlarge the gamma field; the austenitic alloys, stable a t room temperature, also exhibit regularities in behavior, both with respect to properties and manufacture. '

Apart from the foregoing classification, it is possible for alloys containing carbon to form another large group comprising the elements that

Tempering temperature in deg. Cent Iron-Cementite Diagram

Loss rn hardness in a lo'o carbon steel, due to tempertng

FIG. I.-IRON-CARBON SYSTEM AS EXAMPLE OF SYSTEM WITH ONE ELEMENT POSSES- SING TWO MODIFICATIONS, WHICH RESPECTIVELY EXHIBIT DIFFERENT SOLID SOLUBILI- TIES FOR A SECOND ELEMENT OR COMPOUND.

form special carbides, such as chromium, molybdenum, tungsten, vanadium, titanium, tantalum, etc. The similarity that exists between these elements is shown by the fact that all form a closed gamma field in the absence of carbon. Therefore it would appear desirable to examine the influence of several of these carbide-forming elements; a particularly suitable field for these investigations appears to be that of the phenomena occurring during the operations of hardening and tempering. Two possibilities obtain in the hardening of alloys:

1. The solvent metal possesses two modifications; the solute element dissolves to different degrees in these two modifications. Upon transformation on cooling from the phase type stable a t a high temperature to that stable a low temperature, a new phase may not be given the oppor- tunity to separate freely, and tempering of the quenched alloy leads to a

E. Houdremont: Stahl und Eisen (1930) 60, 1517; Rev. de Met. Reference of footnote 1.

E. HOUDREMONT, H. BENNEK AND H. SCHRADER 3

series of changes in properties similar to those shown in Fig. 1. Example: The Fe-C system.

2. The solvent metal possesses but one modification; the solubility of the solute element in this modification increases upon increasing temperature. Rapid cooling from a high temperature will retain a supersaturated solid solution, which upon tempering will precipitate a

-4 OIo W

Iron- Tungsten Diagram (after Takedal

Br~nell Hardness 4001 [ - . - I I I 1 1 1

500 600 700 800 900 1000 1100 1200 1300 1400 1500 Tempering temperature in deg. Cent.

Variation of hardness in quenched Iron- Tungsten alloys due to tempering (after Sykes)

FIG. 2.-IRON-TUNGSTEN SYSTEM AS EXAMPLE OF POSSIBILITY OF HARDENING DUE TO DECREASING SOLUBILITY WITH DECREASING TEMPERATURE AND SUBSEQUENT PRE- CIPITATION OF CONSTITUENTS FROM SUPERSATURATED SOLUTION.

new phase and cause hardening, as shown in Fig. 2. Examples: The solution of Fe3C in a Fe; the Fe-Mo and Fe-W systems.

There is no reference in the literature to a possible combination of these two processes, caused by the presence of both an allotropic transformation, and also an increasing solubility of a separate phase with increasing temperature. Such a changing solubility is exhibited by austenite and iron carbide in the iron-carbon system, shown by the line ES.

TABLE 1.-Precipitation-hardening of Austenitic Chromium-nickel Steel Induced by Tempering

The existence of the line ES is revealed in the hardening of Fe-C alloys by the fact that hypereutectoid steels quenched from temperatures above the line ES exhibit an increased depth of hardening and a greater

Tensile Strength. Kg. per Sq. Mm.

85 90

C Si M n Cr Ni Wo 0.50 1.80 0.75 15 13 2

. . . . . . . . . . . . . . . . . . . . . Water-quenched from 1 2 0 0 " .

. . . . . . . . . . . . . . . . . . Subsequently tempered at 7 0 0 " .

Yield Point, Kg. per Sq. Mm.

33 50

4 HARDENING AND 'I :RING O F STEELS CONTAINING CARBIDES

amount of retained austenite, owing to the increased solubility of the carbide. In tempering, however, no special effect is to be expected when quenching has been performed from above the line ES, instead of from a temperature just above the eutectoid horizontal, since the new phase is the same in both cases; namely, Fe3C.

Although the increasing solubility of Fe3C in austenite with increasing temperature exerts an influence on the mechanism of hardening but not on that of tempering, such is not the case in the presence of elements forming special carbides in addition to iron carbide (as, for instance, carbides of tungsten, vanadium, titanium, chromium, etc.). These carbides have special solubili- ties in the austenite field, shown by special solubility curves, so that after transformation is complete the special carbide will progressively go into solution with increasing temperature. In purely austenitic steels precipitation-hardening occurs by quenching and tempering through the action of such carbides, as shown in the unpublished investigations by Houdremont and Kallen (Table I), and in the more recent work of G r e u l i ~ h . ~

ln:regard to high-tungsten steels, i t can further be shown that so long as quenching is carried out from normal temperatures (about 760" C.) the depth of hardening decreases with increasing tungsten content, owing to the abstraction of carbon from the matrix through the formation of tungsten carbides which are insoluble a t normal quenching temperatures. But with increasing quenching temperatures the depth of hardness increases and from a certain quknching temperature upward exceeds the maximum depth obtainable with plain car-

3 Greulich: Archiv j. d . Eisenhuttenwesen (1931132) 6, 323.


bon steels (Fig. 3). Thus, by alloying a steel with tungsten and quenching from a low temperature, it is possible to produce a steel with a very thin hardened layer. With increasing quenching temperature and conse- quently increasing proportion of carbon dissolved in the form of the stable carbide, greater depth of hardness will be secured.

At the same time steels containing such carbides exhibit great indiffer- ence to overheating. Whereas plain carbon steel, quenched from a temperature only 60" higher than its normal quenching temperature, develops a coarse grain, steels containing such carbides will not show exaggerated grain growth when subjected to considerably higher temperatures because the finely dispersed carbide inhibits the growth of the austenite grains.

Since the solution of these carbides increases in amount with rising temperature and in some instances becomes complete only near the solidus curve, it is of interest to ascertain whether the separation of the carbides during the tempering process nevertheless takes place at the same temperatures as iron carbide. Observations available at present appear to indicate that the temperature at which precipitation takes place depends upon the temperature of solution (Table 2).

TABLE 2.-Relation between Quenching and Precipitation Temperatures in Various Systems Capable of Precipitation-hardening

System Quenching Precipitation

Temperature, Temperature, ( Deg. C. 1 Deb. C.

On studying the tungsten steel mentioned above during the tempering operation, it is found that an increase in quenching temperature and therefore in the quantity of dissolved carbide increases the stability of hardness upon tempering (Fig. 4). Whereas no essential difference is to be found between tungsten steel and carbon steel when quenched from normal temperatures, tungsten steel when quenched from higher temperature exhibits a greater stability of hardness upon tempering than carbon steel. This cannot be ascribed to an increased retention of austenite and conse- quent transformation of this residual austenite upon tempering, because repeated tempering a t increasing temperatures would progressively decompose the residual austenite into martensite, which in turn would soften, not harden; all samples in the present study were tempered eight

Duralumin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . fron-beryllium.. . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . Iron-titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron-tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Iron-molybdenum similar). . . . . . . . . . . . . . . . . . . . . . . . . . . .

About 500 1000-1200

1300-1400

'Mom 456-600

700-900


times. It is reasonable to suppose that the precipitation of tungsten carbide occurs progressively on tempering a t higher temperatures and that this is the cause of the improvement in the stability of hardness upon tempering. Since tungsten forms either complex iron-tungsten carbides or the stable tungsten carbide WC, according to the heat treatment to which the steel is subjectedJ4 further experiments with tungsten steels were not performed.

Instead of pursuing the study of tungsten steels, a number of vanadium-iron-carbon alloys of varying composition were investigated.

- Water-quenched from BOO0 C ----- 9, .. loooO C -.-.- ,, .. ,, 1200° C

In view of the results of Maurer6 and of Vogel and MartinJ6 i t may be

L5"io Carbon Steel Steel containing 1.S0lo C 8 8 W Rock well Hardness, C Scale Rockwell Hardness, C Scale

70

60

50

40

30

taken that only a single carbide of very low solubility occurs in the vanadium-iron-carbon alloys of industrial importance, so that complica-

20

tions due to the disintegration of metastable compounds need not be feared.

20

From the investigations of HougardyJ7 i t appeared that vanadium carbide is practically insoluble and that therefore i t will not dissolve in solid solution. He concluded that part of the total carbon is combined

10

0 100 200 300 400 500 600 700 100 200 300 400 500 600 700

Tempering Tempeiature in deg. Cent. Tempering Temperature in deg. Cent. FIG. 4.-INCREASE OF STAEIILITY IN HARDNESS UPON TEMPERING IN TUNGSTEN

STEEL, DUE TO INCREASE OF QUENCHING TEMPERATURE. ABSENCE OF THIS PHE- NOMENON IN A PLAIN CARBON STEEL OF OTHERWISE IDENTICAL COMPOSITION.

with vanadium and thus takes no part in the transformations occurring in the steel; however, the electrical resistance measurements carried out with the steels investigated by Hougardy show that vanadium carbide does actually pass into solution a t higher temperatures, a fact that had

F. Polzguter and W. Zieler: Stahl und Eisen (1929) 49, 522. 6 Maurer: Stahl und Eisen (1925) 46, 1629. 6 Vogel and Martin: Archiv f. d. Eisenhuttenwesen (1930/31) 4, 487.

Hougardy: Archiv f. d. Eisenhuttenwesen (1930/31) 4, 497.

E. HOUDREMONT, H. BENNEK AND H. SCHRADER 7 '

already been established by Maurer by means of physical measurements on vanadium-carbon steels.

A special influence exerted by vanadium carbide is evident from the unpublished investigations carried out in 1911 by F. Rittershausen, who found that vanadium steel exhibited different properties according to the temperatures from which it had been quenched. While no effect of the vanadium on the properties of steel could be detected when quenching from normal quenching temperatures, an increase in quenching temperature produced an important change in properties, as discussed below.

The range of steels investigated by the present authors is given in Table 3 .

TABLE 3.-Chemical Composition of Vanadium Steels Studied

1 Composition, Per Cent 1 Steel C s i

Mn I "

I OV0.4. . . . . . . . . . . . . . . . . . . . . . . . OV0.5. . . . . . . . . . . . . . . . . . . . . . . .

1st series OV0.9. . . ... . . . . . . . . . . . . . . . . . . . OV1.6. . . . . . . . . . . . . . . . . . . . . . . . ov2 .2 . . . . . . . . . . . . . . . . . . . . . . . .

4th series { 35V0.4. . . . . . . . . . 36V0.6. . . . . . . . . .

The influence of vanadium and carbon on the Brine11 hardness in the annealed and the normalized (air-cooled from 900" C.) conditions is shown

in Fig. 5. In both conditions there is a tendency for the hardness to decrease with increasing vanadium content. This, of course, is to be explained by the abstraction of carbon from the matrix owing to the formation of globular vanadium carbide.

Brinell 'Hardness 1013000 Brinell ' Hardness 1013000

0 2 4 6 O I o V

Annealed

FIG. 5.-HARDNEBB OF ANNEALED AND NORMALIZED CARBON-VANADIUM STEELS.

FIG. 6.-EFFECT OF VANADIUM ON QUENCH HARDNESB OF CARBON BTEEL.

The variation in hardness brought about by quenching in water from different temperatures is given in Fig. 6. With low carbon contents, a decrease in hardening power is again shown for increasing vanadium content, while a t the same time an increase in quenching temperature


results in a perceptible increase in hardness (contrary to the behavior of vanadium-free steel). The clearest results are those obtained with steels containing 1 per cent of carbon. At low quenching temperatures, the hardness decreased considerably with increasing vanadium content, while a t temperatures in the range of 1250" to 1300" C. the same hardness was obtained as in plain carbon steels quenched from normal temperatures. Steels that lie outside the gamma region, owing to their high vanadium content, showed only a slight increase in hardness due to quenching.

I t is interesting to note that alloys that are purely ferritic, owing to their low carbon content and high vanadium content, still showed an increase in hardness upon quenching of 50 to 60 Brine11 numbers, a

Quench~ng Temperature m deg. C

1250 1200

1100

1000

900

800

700

6OO0 -

1 2 3 4 5 6'10 V , FIG. 7.-TEMPERATURE RANGES FOR OBTAINING VARIOUS DEGREES OF HARDNESS

IN VANADIUM STEELS CONTAINING 1 PER CENT C, DETERMINED BY APPEARANCE OF FRACTURE AND BY HARDNESS TESTS.

phenomenon also observed by Maurer when experimenting with a steel that contained 1 per cent of carbon and 10 per cent of vanadium.

These quenching experiments should prove that i t cannot be assured a priori that the vanadium carbide has no part in the process of hardening; on the contrary, with increasing temperature the vanadium carbide dissolves in solid solution, as shown by Maurer, and becomes an effective hardening agent. Only on this assumption can be the great hardness of steel containing 6 per cent of vanadium and 1 per cent of carbon be explained. The solution of the vanadium carbide in the austenite was confirmed by measurement of the specific electrical resistance.

Fig. 7 illustrates the exact hardening conditions for vanadium steels containing 1 per cent of carbon as a function of the quenching temperature and the position of the Ac3 transformation. A noteworthy feature is the interval of temperature-increasing with the vanadium content-through which the vanadium steels must be heated above the A c ~ point before the slightest hardening effect-even in incomplete form-sets in. In order to obtain optimum hardness, it is necessary to raise the temperature

10 HARDENING AND TEMPERING O F STEELS CONTAINING CARBIDES

F I ~ . 8.-MICROSTRUCTURE OF QUENCHED VANADIUM STEELS CONTAINING 1 PER CENT CARBON.

a. Steel containing 1 per cent carbon, water-quenched from 740" C. b. Steel containing 1 per cent carbon and 1 per cent vanadium, water-quenched

from 900" C. c. Steel containing 1 per cent carbon and 3 per cent vanadium, water-quenched

from 1100" C. d, e and f. Steel containing 1 per cent carbon ancl 6 per cent vanadium. d. Water-quenched from 900" C. e . Water-quenched from 1100" C. f. Water-quenched from 1300" C.

Etched with 3 wer cent nitric acid.

E. HOUDREMONT, H. BENNEE AND H. SCHRADER I i

even further. Microscopic examination shows that the region of imper- fect hardening is characterized by the presence of a considerable quantity of undissolved vanadium carbide, which exerts an intensifying influence on the tendency to transformation during the hardening process and thus acts as nuclei in facilitating the formation of troostite. At temperatures up to 900" C., and even up to 1100" C., depending upon the vanadium content, the grain boundaries exhibit remnants of troostite, the greater part of which is in direct connection with accumulations of carbide (Fig. 8). The photographs of the steel containing 6 per cent vanadium

2 Per cent Vanadium

FIG. Q.--&UENCHING TEMPERATURE RANGE FOR VANADIUM STEELS CONTAINING 1 PER CENT CARBON.

and I per cent carbon clearly show the dissolution of the vanadium carbide with increasing temperature. It will also be noted that the structure assumes a hardenitic appearance, whereas plain carbon steels form a coarse acicular martensite upon quenching from 900" C. Vanadium steel quenched from high temperatures possesses a structure closely resembling that of high-speed tool steel.

In vanadium steels, as in tungsten steel, the embedded carbides also render the steel less sensitive to overheating; in other words, the temperature range available for successful hardening is increased (area in Fig. 9). This is shown very clearly in steels low in vanadium, where iron carbide is present in sufficient quantity to produce full hardness within a fairly narrow temperature range above A c ~ . In the case of high vanadium contents optimum hardness is not obtained until larger quantities of vanadium carbide have gone into solution; accordingly


the range of temperatures from the best hardening temperature to the overheating temperature is not larger than for plain carbon steels.

As a consequence of the abstraction of carbon from the matrix through the formation of vanadium carbide and of the inoculating effect of the vanadium carbide upon the transformation, vanadium has the further effect of reducing the depth of hardening obtainable in quenching from normal temperatures. This is proved by a comparison of a steel containing 1 per cent carbon and 1 per cent vanadium with a steel containing 1 per cent carbon but no vanadium. This result is in agreement with the findings of other^.^

Since plain carbon steels and vanadium steels-harden throughout a cross-section of 30 mm. on a side, and since increase in quenching temperature increases the depth of hardening, it is necessary in studying the

Quenching temperature in O C. FIG. 10.-SCHEMATIC REPRESENTATION OF DEEP-HARDENING CAPACITY OF STEEL

CONTAINING 1 PER CENT CARBON AND 1 PER CENT VANADIUM, AS COMPARED WITH PLAIN CARBON STEEL.

effect of high quenching temperatures in vanadium steels to use a larger section; a section of 80 mm. on a side was adopted and found satisfactory. Quenching temperatures from 1000" C. upwards increase the capacity of vanadium steel for deep hardening, yet despite these high temperatures vanadium steel quenched from temperatures up to 1100" C. showed an extremely fine-grained fracture. This influence of vanadium is represented schematically in Fig. 10. These observations on vanadium- bearing steels thus confirm the observations on tungsten steels, and justify the conclusion that in steels containing such special carbides an increase in hardening power and a material change in the critical rate of cooling in comparison with plain carbon steels is afforded, entirely because of the solution of the special carbides.

It is still necessary to inquire how the solution of vanadium carbide at high quenching temperatures affects the stability of hardness on temper-

. ' ing. When quenched from below A c ~ and even from 800" C., the tempering curves of vanadium steels do not differ materially from those of plain

Schilling: Kruppsche Monatshefle (1923) 4, 123; Stahl und Eisen (1923) 43, 1048.


carbon steels. But vanadium steelsquenchedfrom 900" C. and tempered a t about 600" show a slight anomaly in the course of the hardness-temperature curves (Fig. l l ) , which becomes quite conspicuous when the steel is quenched from 1000" C:(Fig. 12).

In order to make sure that the increase in hardness upon tempering a t 600" C. could not be ascribed to a specific effect of thevanadium itself, a series of specimens practically free from carbon were included in the investigation. As expected, these specimens showed that the hardness is in no way dependent upon the quenching or tempering temperatures. But with as low a carbon content as 0.05 per cent an increase in hardness was noted between 550" and 650" C. tempering temperature in steels that,

r41~ c BnmN Hardness

rq3aOO

O I 5 % C BnneN Hardness

IOI3WO

FIG. 11.-VARIATION OF HARDNESS PRODUCED BY TEMPERING VANADIUM STEELS PREVIOUSLY WATER-QUENCHED FROM 900' C.

because of the vanadium and carbon content, lie within the gamma region. This occurs also for steels containing 0.15, 0.35 and 1 per cent carbon. In low-carbon steels (0.05 and 0.15 per cent), these phenomena persist in approximately the same degree a t quenching temperatures up to 1300" C.

High-vanadium steels with a higher carbon content assume a marked hardness only a t quenching temperatures from 1100" C. upward. Conse- quently, the tempering phenomena also make their appearance only after quenching from these high temperatures. The stability of hardness upon tempering increases with the quenching temperature and the proportion of dissolved vanadium carbide (Fig. 13). This effect of the vanadium makes itself felt even with very low vanadium contents, such as are sometimes encountered in eutectoid tool steels (Table 4). At temperatures up to 400" C. there is practically no difference in hardness between plain carbon steels and vanadium steels. Even a t higher tempering temperatures vanadium steels quenched from 800" C. scarcely

Brinell Hardness 1013000

Brinell Hardness 1013000

FIG. 12.-VARIATION OF HARDNESS PRODUCED B Y TEMPERING VANADIUM STEELS PREVIOUSLY WATER-QUENCHED FROM 1 0 0 0 ° C.

BrtnellHardness to/3wo

700

6W

500

400

300

200

0 200 400 600 Tempertng Temperature w, deg. C

Water-quenched from tlOOo C.

Br~nell Hardness 1013000

700

600

500

400

300

200

0 200 400 600 Tempenng Temperalum In deg. C Weter-quenched fmm 1300° C

FIG. 18.-INCREASE OF STABILITY I N HARDNESS UPON TEMPERING I N VANADIUM STEELS CONTAINING 1 PER CENT CARBON, DUE TO INCREASE I N QUENCHING TEMPERA- TURE. '

E. HOUDREMONT, H. BENNEK AND H. SCRRADER 15

differ from carbon steels. Only with highest quenching temperatures will steels tempered at 550" to 650" C. show an increase in hardness corresponding to that revealed by the other investigations. As there was reason to surmise that the time factor is of importance for the solution of the carbides, the time of retention at the quenching temperature was increased up to 10 hr. without any change in results. Therefore it must be concluded that heating to quenching temperature for the normal time, about 10 minutes, suffices to induce a state of equilibrium in the solution of the vanadium carbide.

Since the tempering effect observed in vanadium steels might be attributed not to a precipitation of the vanadium carbide but to a trans-

TABLE 4.-Increase of Stability in Hardness in 1 Per Cent Carbon Steel upon Tempering, Due to Increase of Quenching Temperature and Slight

Addition of Vanadium.

, formation of residual austenite, such as is known to occur in high-speed tool steel when tempered at about 600" C., all the steels studied were subjected to measurement of magnetic saturation, for the purpose of determining their austenite content (Fig. 14). The steels were quenched from temperatures a t which the precipitation effect was already clearly noticeable, and afterwards tempered at temperatures varying from 100" to 600" C. The saturation measurements were carried out on cooled specimens in a field of 10,000 gauss, in comparison with the same steels annealed at 750" C. for 3 hr. and cooled in the furnace.

The amount of residual austenite in the vanadium-bearing steels is lower than in plain'carbon steels, owing to the enhanced tendency towards transformation induced by the numerous carbide particles still present at the quenching temperatures. The series of low-carbon specimens

Waterquenched 67-69 from 1000 O C. 54 46-47 3-0 32-34 211

shows practically no residual austenite, tliough a distinct increase in hardness sets in on tempering. The essential point is that the disintegration of the residual austenite occurs a t the same temperatures as for plain carbon steels; viz., about 300" C. From this temperature on, all steels possess the complete saturation of the annealed state. A transformation of residual austenite into martensite therefore cannot account for the secondary increase in hardness observed in vanadium steels upon tempering a t about 600" C. As mentioned above, such a possibility is also

Magnetlc Saturabon 4 Z I - in gauss for a held Intensity of 10000 gauss

23000

22200

2 1400

20600

19800

19000

18200,+

17400

r ~ ~ o o

15800/

150000 at 75P0 C.

200 400 600 800 Tempering Temperature ln deg C

Cooled ln furnace

FIG. 14.-VARIATION OF MAGNETIC SATURATION.

prohibited by the fact that a vanadium steel that after having been tempered for one or more times, for example at 550" C., exhibiting there- upon the secondary increase in hardness, can be subsequently tempered a t 600" C. producing a final hardness identical with that produced by the first tempering at 600" C. Any martensite newly formed from residual austenite would suffer a tempering effect under such circumstances; i.e., a softening.

It might still be objected that vanadium steels on being quenched retain a certain amount of residual austenite, which during the tempering process precipitates carbides that increase the hardness or enhance the stability of hardness on tempering, without changing the residual austenite (impoverished in carbon) into martensite. This, of course, would not produce an increase in the value of magnetic saturation. Rowever, the formation of an austenite of a stability which when annealed at 750" C. would not be changed into alpha iron is inconsistent with the

.'-

7

1 !

.i I /..---. *-.

svo- - - 5 vo.31 35 VO - 35 V0,91 10001 Water-quenched

n---=lo0 V 0 --- 100 V3 flOO/ Water-quenched

~nnkaled


general behavior of vanadium steels in hardening. There is nothing whatever to show that vanadium has a stabilizing effect on the gamma solid solution. Such a stabilization of austenite is particularly incon- ceivable in the case of the low-carbon alloys that possess very slight hardening power. It can further be shown that the degree of saturation actually found in the steels investigated agrees so closely with the saturation calculated, when allowance is made for the reduction of saturation due to the non- magnetic alloying elements, that an appreciable amount of aus- a tenite cannot possibly be present. Attention is further drawn to the fact that all precipitations known in austenite and austenitic steels, capable of producing a perceptible hardening effect, occur a t much higher temperatures (700' to 800" C.).g Finally, i t would be necessary to assume that vanadium steels form two different types of residual austenite, one of which changes into martensite in the usual way a t tempering tempera- b tures up to 300" C, and another that possesses extraordinary stability. All this evidence indicates that the possibility of a transformation of residual austenite on tempering can safely be left out of consideration. The increase in

FIG. 15.-PRECIPITATION O F CARBIDE, l-laKlness that occurs in steels con- .I,,BLE UNDER MIc,o,,oPE. I, S T E E L CON-

taining vanadium carbide when TAINING 1 PER CENT CARBON AND 3 PER CENT tempered a t 600' C. must there- VANADIUM.

fore be ascribed to a change in the nature of the martensite or the alpha iron, respectively.

Metallographic observations, which demonstrated the progressive solution of the vanadium carbide a t higher temperatures, also revealed the precipitation of an extremely finely dispersed constituent on tem-

% B. Strauss, H. Schottky and J. Hinniiber: Zlsch. f. anorg. u. allg. Chem. (1930) 188, 309.

A. Bennek and P. Schafmeister: Archiv f. d. Eisenh.ullenwesen (1931132) 6, 615. W. Koster: Archiv f. d. Eisenhzillenwesen (1932133) 6 , 17.

pering. It is true that precipitation can only be observed metallographi- cally in the case of prolonged tempering at about 750" C.; in other words, a t a temperature from 100" to 150" C. higher than the temperature of maximum increase in hardness. Fig. 15 shows two unetched sections of the specimen containing 1 per cent carbon and 3 per cent vanadium, in the quenched state and after having been tempered a t 750' C., respec-

PIG. 1G.-MICROSTR~CTURE OF STEEL COXTAINING 3 PER CENT VANADIUM AND 1 PER CExT CARBON, I N TEMPERED CONDITION.

a. Water-quenched from 100" C., tempered 1 hr. a t 500" C. b. Water-quenched from 1100" C., tempered 1 hr. a t 600" C. c. Water-quenched from 1100" C., tempered 1 hr. a t 750' C. d. Water-quenched from 1100' C., tempered 3 hr. a t 750" C.

Etched with 3 per cent nitric acid.

tively. The retarded precipitation of the vanadium carbide also delays the disintegration of the martensite structure, as apparent in Fig. 16.

That the phenomena on tempering are due to a kind of precipitation- hardening is also proved by the hardness-tempering time curves repro- duced in Fig. 17. Whereas a t 500' C. tempering temperature, the hardness increases up to 100 hr. exposure to that temperature, i t decreases after 100 hr. exposure to a tempering temperature of 600" C., and more quickly if the tempering temperature be increased to 650" C. In the

E. EOUDREMONT, H. BENNEK AND H. SCHRADER 19

steel of very low carbon content, precipitation-hardening sets in in the course of the very first hour.

The whole of the investigations described so far would seem to prove beyond dispute that in the hardening of vanadium-carbon steels the vanadium carbide that dissolved a t higher temperatures does not, in spite of the gamma-alpha transformation having occurred, precipitate from the alpha solid solution a t temperatures below 500" to 600" C., and

Brine11 Hardness 10/3000 5 V0.3

0 2 4 6 8 1 0 100 Duration of tempering in hrs.

FIG. 17.-INFLUENCE OF DURATION OF TEMPERING ON HARDNESS OF LOW-VANADIUM STEELS.

that when it does precipitate a t proper tempering temperatures it causes an increase in hardness, which imparts to the steel the property of resistance to tempering.

The observed stability of hardness upon tempering due to the presence of vanadium, and its dependence upon variation in the quenching temperature (which must be regarded as a specific effect of the vanadium carbide) must also be reflected in the mechanical properties of the steel. The stability of the high tensile strength of vanadium-bearing constructional steels on tempering has been mentioned by Putz,lO Hohage and Grutzner,ll and others.

10 Putz: Mdallurgie (1906) 3, 635. 11 Hohage and Grutmer: Slahl und Eisen (1925) 46, 1126.


Since the conception that the special effects exerted by vanadium are caused by precipitation of vanadium carbide is entirely new, it appeared advisable to perform additional experiments upon these steels. Because of the low vanadium and carbon contents present in constructional steels, these investigations were mainly limited to steels containing, respectively, 0.05 and 0.35 per cent carbon, with vanadium contents up to. 0.4 per cent. Since earlier researches, dealing in particular with copper and titanium, had shown that a certain amount of precipitation-hardening often increases the yield point rather than the tensile strength, the ratio of the yield point to tensile strength was takenpas a measure of the amount of precipitation-hardening occurring. Fig. 18 clearly shows the influence of precipitation within the temperature range from 500" to 600" C. Whereas the vanadium-free steels exhibit the usual changes in mechanical properties on tempering, the influence of the carbide precipitation in the vanadium-bearing steels is clearly discernible. In the case of the 5 V 0.2 steel the yield point curve clearly indicates a rise above 500" C. (after an initial decrease between 300" and 500" C.), and the tensile- strength curve also shows an increase in this temperature.range, though not so pronounced; but the yield ratio shows a pronounced increase. The effect is still more marked in the 5 V 0.3 steel.

In the case of the 35 V 0.2 and 35 V 0.4 steels, which are richer in carbon, the loss in tensile strength a t 300" to 500" C., due to the coagula-

TABLE 5.-Effect of Duration of Tempering on Tensile Properties of Vanadium Steels Tempered at 600" C.

tion of the iron carbide, is more pronounced, in accordance with the greater proportion of this constituent. Nevertheless, here again the influence of vanadium carbide precipitation, particularly on the yield point and on the yield ratio, is clearly marked. With precipitation beginning a t 500" C., there occurs a decrease in ductility, as may be seen

Steel

5 V 0 . 3

35V0.4

Heat Treatment

Oi l -queqched from 950" and tempered 1 hr. at 600' C . . . . . .

8hr.at600°C . . .

Oi l -quenched from 950" and tempered 1 hr. at 600' C . . . . . . .

8 hr. at 600" C . . .

Yield Point,

2~:. --

43 48

91 86

Tensile Strength,

2.g:.

47.7 57.1

101.4 99.9

Elongation, (1 = Sd) Per ~ e n ;

29.0 26.4

18.4 18.2

z:;!:, Per Cent

90.3 84 .4

89 .8 86 .1

Reduction in Area. Per Cent

79 78

57 55

*-• Tensile Strength -.-.-.- Yield Point - Yield Ratio 100 -.-r-.-. Elastic Limit -------- Elongation ---9 Reduction of area

ido 2ho 3b0 4b0 5d0 6d0 7i10

Tempering Temperature in deg. Cent. Tempering Temperature in deg. Cent. Tempering Temperature in deg. Cent.

FIG. 18.-VARIATION IN TENBILE PROPERTIES OF LOW-VANADIUM STEELS DUE TO VARIATION IN TEMPERING TEMPERATURE.

22 HARDENING AND TEMPERING O F STEELS CONTAINING CARBIDES

most clearly in the 35 V 0.4 steel. The change in tensile properties following an increase in tempering time at the same temperature is in agreement with the isothermals in Fig. 17.

Table 5 gives the different results obtained by tempering the 5 V 0.3 and 5 V 0.4 steels for periods of 1 and 8 hr. Whereas in the case of the steel with the higher carbon content the tensile strength and yield point have already passed through their maximum after the lapse of 8 hr., the low-carbon steel does not show this behavior, a fact that is in accord with the hardness results. The impact strength (as measured by means of the Mesnager specimen) of the same two steels after different heat treatment is given in able 6 . The figures can be regarded as satis-

TABLE 6.-Impact Strength of Tempered Vanadium Steels KG-M. PER SQ. CM. AFTER OIL-QUENCHING FROM 950' C. FOLLOWED BY TEMPERING

Mesnager specimen 10 X 10 X 55 mm., area of fracture 10 X 8 mm.

1 Hr. at 500"

I 5V0.3 . . . . . . . . . . . . . . . . . 30 I 35V0.4 . . . . . . . . . . . . . . . . . 10.2

factory, although the values usually associated with high-grade constructional steels of the same tensile strength were not attained in the

T ~ L E 7.-Influence of Sectional Area on Variation of Tensile Properties Induced by Precipitation of Carbide in Vanadium Steel

STEEL 35 V 0.4 OIL-QUENCHED FROM 1000' AND TEMPERED 100 HR. AT 500'

8 Hr. at 600'

22.6 8.5

- 1 Hr. at 600"

30 7.9

case of the 35 V 0.4 steel. I t is, of course, well known that precipitation phenomena are generally accompanied by a decrease in impact strength. This is true also of the precipitation of vanadium carbide, as conclusively proved by the further experiments on the 35 V 0.4 steel (with specimens of various cross-sectional area, with a view to ascertaining at least within certain limits the influence of cross-sectional area).

Table 7 sets forth the results obtained with specimens of 20, 40 and 60 mm. dia. These specimens were oil-quenched from 1000" C. and

1 Hr. at 650'

29 12.1

Diam- eter. Mm.

20

40

60

Yield

:Fir Sq. &m.

104

92 90

89 85

tion by Forging

49 fold

12.2fold

5.5 fold

Tensile sz:z$ Sq. Mm.

112

105.9 104.8

-------- 104.3 102.7

Position

O k E ~ ~ t ----

---- Outside Inside

Outside Inside

pl%!~, " Cent

93.0

87.0 91.0

85.3 14.8 45 2 . 2 82.8 1 13.4 1 40 1 1.5

Elonga-

3). Per Cent

----

12.7 ----

14.7 16

Reduc-

oti:;a, Per Cent

46

50 49

&:%:, Kg-m. per

sq. (lo 10 X 55 Mm.)

4.0

2.3 1.7

TABLE 8.-Efect of Addition of Vanadium in Alloy Steels. Increasing Resistance to Tempering with Increasing Quenching Temperature.

. (From researches by Rittershausen)'

I 31 I Heat Treatment I~ ie ld ~oint / l~ensi le strength/( Eloncration I Reduction 11mDact ~trenclth/l 0

Chromium Steel containing C Si Mn Cr 0.29 0.14 0.15 1 per cent

Chromium-vanadium Steel containing C Si Mn Cr V

0.26 0,10 0:13 0.96 0.27 per cent

Chromium-nickel Steel containing C Si Mn Ni Cr 0.31 0.06 0.29 4.10 b17 per cent

Chromium-nickel- Vanadium Steel containing

C Si Mn Ni Cr V 0.29 0.24 0.26 4.34 1.52 0.48 per cen,

Quenched from

850°

Quenching Medium

Oil

Oil

Oil

Oil

Water

Water

Water

Water

Water

Water

Water

Water

. . 4 Tempered kg./sq.mm. kg./sq.mk (1 - 5 d)/O/o of area/Oio kg.m./sq.c. a at *) 'A

m 600 O 44 70 25.2 66 23.8 8 600° 46 70 18.3 64 18.7 .?

tempered at 500' C. for 100 hr., in accordance with the maximum effect indicated by the isothermals in Fig. 17. As appears from Table.7, the yield ratio for the 60-mm. dia. section is slightly lower than for the smaller sections. The notched-bar impact value is very low. For plain vanadium-carbon steels, with cross-sections of 60 mm. dia. and upwards, it would be necessary to resort at least to water-quenching in order to secure a uniform effect throughout the cross-section.

The theories evolved in connection with plain carbon-vanadium steels in explanation of the effect of the vanadium carbide are directly applicable to complex constructional steels. This is apparent from the results obtained in 1911 by F. Rittershausen, which led the authors to under- take the present work. Some typical examples of Rittershausen's results are given in Table 8.

The chromium steels as well as the chromium-nickel steels plainly show that when quenched from normal temperatures the vanadium content exerts no effect upon the mechanical properties; on the contrary, owing to some of the carbon combining with the vanadium, there is a tendency for the vanadium-bearing steels to be somewhat softer. When higher quenching temperatures are employed, the vanadium-free steel remains on the whole unaffected, whereas the vanadium-bearing steels exhibit the marked effect of the vanadium in increasing the tensile strength, yield point, etc., upon tempering. At the same time, the impact values fall off considerably, owing to the precipitation of vanadium carbide.

In chromium-molybdenum steels, which of themselves show considerable resistance to tempering, maintaining high tensile strength and yield point, etc., on tempering a t high temperatures, an addition of vanadium coupled with an increase in quenching temperature produces a similar improvement in the mechanical properties (Fig. 19). When such a steel is quenched from a lower temperature, the influence of the molybdenum alone makes itself felt on tempering a t 550' C. Only with increasing quenching temperature does the effect of the vanadium appear also on tempering at 600' C.

From the above we may conclude that an addition of vanadium to constructional steels will produce no effect on the mechanical properties unless the quenching temperature is raised so as to utilize the influence of the vanadium carbide. The only effect of an addition of vanadium to constructional steels quenched from normal temperatures is to produce a fine grain, owing to the retardation in grain growth in the austenite by carbide particles. The h e r grain of vanadium steels causes a higher ratio of yield point to tensile strength. The effectiveness of vanadium as a deoxidizer is well known and need not be discussed here.

Particular interest is attached to a study of vanadium steels treated in various ways, with respect to their strength a t elevated temperatures.


P. Promper and E. Pohll2 some years ago called attention to the mainte- nance of strength a t elevated temperatures exhibited by vanadium- alloyed mild steel, and proposed the use of such material for boiler con- struction. I t has been known for some time that this improvement

Yield Point in kg.lsq.mm. Elongation & Reduction

of area in per cent

I40 -

120

100

Elongation Reduction 01 area

201 /#++++-++q O 450 550 600 650 700 750

Tempering Temperature m deg. C Impact Strength in ha.-m./sa.cm.

Test prece 10x10 155 mm [lengthwise) Test piece 10110 x55 mm Ilenothwtsel

500 550 600 650 700 750 Tempering Temperature ln deg. C.

-VARIATION OF TENSILE PROPERTIES OF CR-MO-V STEEL WITH VARY TEMPERING TEMPERATURES.

'ING

in the mechanical properties cannot always be obtained. I t is reasonable therefore to suspect that increased retention of strength at elevated temperatures can be obtained only under certain definite conditions induced by heat treatment, where the precipitation of the vanadium carbide plays a part.

Fig. 20 shows the tensile values of variously treated low-carbon vanadium steels tested at 500" and 600" C. The steel that has not been tempered a t all, and that which had been tempered at 600" C., exhibit higher tensile values under the temperature conditions of the test. Of the two, the one that had not been tempered is bound to exhibit the

1". Pr6mper and E. Pohl: Archiv f. d. Eisenhtiitenwesen (1927128) 1, 785.

more pronounced effect during the tensile test of 20-min. duration, since the precipitation in this case can start only during the tensile test itself. The steel that had been tempered at 700' C. shows a t elevated temperatures tensile properties that do not differ essentially from those found with plain carbon steel, particularly as regards the yield point. Thus it is clear that the tensile strength and the yield point a t elevated temperatures are raised by the precipitation of the special carbide. ~ v i d e n t l ~ this will hold equally well for all other steels that

under similar conditions will form special carbides. In considering the tensile properties of steels of this type at elevated temperatures it will therefore always be necessary to take into account the condition in which they are examined; the as-rolled state, in particular, must be regarded as an indeterminate state. Considering, however, the fact that at 500' C. the precipitation of vanadium carbide does not progress far enough in 100 hr. for a decrease in hardness to occur, it is quite legitimate to claim a lasting increase in high-temperature hardness and strength of vanadium-bearing steels upon suitable treatment.

One of the most important requirements for cutting steels, especially high-speed steel, is that of red-hardness. Vanadium steels of rather

Tensk Strength in kg./sq.mm.

60- x-r 5V0

50

---- - 011-quenched from lWOU C., lempered i hr. at 600° C.. air-coded ---- Oil-quenched from iOOOY C.. lempered i hh a1 700° C.. air-cooled

10

0 100 200 300 400 500 600 Yield Point

N) kg./sq.mm. 50

40

30

20

10

0 100 200 300 400 500 600 Tensile Test Temperature in deg. C.

FIG. 20.-INFLUENCE OF VANADIUM ON TENSILE PROPERTIES AT HIGH TEMPERATURES OF VARIOUBLY. TREATED LOW-CARBON STEELS. . .

&A

- 5V0.4

- Oil-quenched from 1000" C.


high carbon content, possessing characteristics similar to those of tool steel, have therefore been studied with a view to determining their cutting efficiency under definite conditions.

Fig. 21 illustrates the influence of vanadium on the cutting life of tools worked a t two different cutting speeds. Although the 6 per cent vanadium steel does not have the long tool life of the best grades of high- speed steel, the marked effect of vanadium with respect to tool life is nevertheless unmistakable. It must be borne in mind that the vanadium steels studied from this point of view all possessed identical carbon contents (1 per cent); it would be illogical to assume a priori that this

Cuttlng Llfe of tool

Materral machined Cr-NI-Steel treated so as to glve a tensrle strength

of 100 kg /sq.mm

Feed 14 mm Thickness of ch~p 5 mm

Treatment Water-quenched Water-quenched Water-quenched Water-quenched from 780° C from 820° C lrom t250° C lrom 1350' C

Bolted at Roo Bo~led at 150' Tempered a t 3 0 0 ~ Tempered at 500'

Rockwell-C Scale I

BJ /s r 83

Hardness of ch~sel edge E41sa E4'/s5 184

FIG. 81.-EFFECT OF VANADIUM ON CUTTING EFFICIENCY OF PLAIN CARBON STEEL.

carbon content and no other represents the optimum percentage for securing maximum tool performance in combination with all of the various vanadium contents examined. However, the results found are sufficient to demonstrate clearly the influence of vanadium in high- speed steel.

The same laws that govern the influence of vanadium in steels hold good for the other elements that form special carbides. Earlier in this paper it was shown that the behavior of tungsten steels corresponds to that of vanadium steels. Similar phenomena occur in chromium steels. With increasing quenching temperature, steels with rather high contents of chromium and carbon undergo a change in the critical rate of cooling, owing to the progressive solution of the chromium carbides in the solid solution, assuming the character of an oil-hardening or air-hardening steel. At the same time the stability of hardness upon tempering is also increased by the chromium carbides.

According to recent investigations by C. Kiittner and W. Tofaute, a 15 per cent chromium steel heated to 500' C. precipita4es a stable

chromium carbide. It is a well-known fact that chromium steels a t 500" C. also exhibit a stability in hardness upon tempering and even an increase in hardness upon tempering like that found in vanadium steels. Furthermore, this behavior furnishes information on the corrosion resistance of these steels. As is well known, steels containing from 13 to 15 per cent of chromium with 0.5 per cent of carbon show good anti- corrosive properties in the quenched state; but after being tempered at from 500" to 650" C. they possess comparatively low corrosion resistance. This loss in corrosion resistance is probably caused by the removal of

Hardness a large amount of chromium from

; ing the of the steels that Tempering Temperature deg C. form the carbides in question. At

FIG. 22.-DIAGRAM REPRESENTING SUPERPOSED PHENOMENA THAT OCCUR ON least with to some proper- TEMPERING VANADIUM STEEL QUENCHED ties, such a system would appear to FROM HIGH TEMPERATURE. be already definitely established.

The influence of vanadium and of the other carbide-forming elements may, in conclusion, be summarized as follows:

As a consequence of the abstraction of carbon from the matrix by the formation of the special carbide, the hardening power of the steel when quenched from the usual temperatures generally is low, because the special carbide is less soluble in austenite than iron carbide. At higher quenching temperatures, owing to the retarding action exerted by the carbide particles on grain growth, vanadium steels are not susceptible to overheating. The increasing solution of the carbide on increasing temperature diminishes the critical rate of cooling, and accordingly increases the depth of penetration of the hardness. On tempering, the special carbide of low solubility separates from the alpha iron only at high tempering temperatures and, by a sort of precipitation-hardening, increases the stability of hardness of the steel to the tempering treatment. The phenomena involved can be represented in the manner shown in Fig. 22. The martensite formed by the gamma-alpha transformation causes a decrease in hardness during the tempering just as in simple iron-carbon alloys, as shown by curve 1. The precipitation of the special

\

' \ \ .

\ \. -----curve 1 - Decomposrhon ofthe marienstte

-.-.- b r v e 2 - Precipitation hardening caused by special carbides - Curve 3 - Tempering Curve of a Vanadium steel

the matrix, owing to the precipitation of the [chromium carbide, CraC. The finely dispersed state taken by the precipitate may be assumed to contribute to this effect.

It ;may thus be seen that a study of the various carbides and the temperatures and compositions a t which they exist appears to offer a'system of classification from which conclusions may be drawn concern-


carbide at higher temperatures brings about an increase in hardness as shown by curve 2. The combination of the two phenomena gives curve 3, synthesized from curves 1 and 2. This curve 3 corresponds exactly to the one with which we are familiar in connection with high- alloy steel, particularly high-speed tool steel. For such steels, however,

, the secondary increase in hardness can also be ascribed to the transformation of the residual austenite into martensite. This transformation, however, is practically completed after one heating to 550' to 600' C. and subsequent cooling; furthermore, the newly formed martensite would become soft very quickly at high cutting temperatures if it were not hardened by the precipitation of the special carbides. It is safe, therefore, to conclude that the secondary increase in hardness occurring in high-speed steels is due not only to precipitation-hardening but also probably in part to the transformation of residual austenite, whereas the stability of the martensitic hardness up to 600' C., the red-hardness, is exclusively due to the fact that special carbides precipitate and coagu- late at higher temperatures than cementite.

metals technology, 1934, vol. i - december 1934 - t.p. …library.aimehq.org/library/books/metals...

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