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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(4): 39-43

Properties of 3Cr2W8V Die Steel With Striations Processed by Laser

CHEN Ij1.3 , ZHOU Hong", ZHANG Zhi-hui" , REN Lu-quarr'O. School of Materials Science and Engineering, Changehun University of Technology. Changchun 130012.

Jilin. China; 2. The Key Laboratory of Automobile Materials of the Ministry of Education, Jilin University,

Changchun 130025. Jilin. China; 3. The Key Laboratory for Terrain Machinery Bionics Engineering of the

Ministry of Education. Jilin University. Changchun 130025, Jilin. China)

Abstract: The striations on the surface of 3Cr2W8V die steel were processed by laser. The microstructure, hard-

ness, wear resistance and thermal fatigue behavior of the specimens processed by laser were measured. The appear-

ance and mechanism of thermal fatigue crack propagation in the zone processed by laser were observed and discussed.

The results show that the wear resistance and thermal fatigue resistance of materials processed by laser are all better

than those of the unprocessed material. The processed zone by laser plays a role in baffling wearing process and crack

propagation. The pile-nail effect of processed zone is the main factor for improving the wear resistance and thermal

fatigue resistance of material.

Key words: laser processing; die steel; property

Natural biomaterials have some unique proper-

ties and functions and have attracted many research-

ers' interest, including their structures, mechanical

properties, physical and chemical behavior and their

biomimetics. Related researches show that there

are, respectively, convex, reticulate, striate, etc.

shapes on body surfaces of some soil animals, such

as the dung beetle and mole-cricket, etc. Researches

in tribology and bionics show that this kind of non-

smooth biont surface can break one consecution andlower the resistance of friction in crawling process,

so eases consumedly the harm to the body surface,thus their wear resistance is superior'{r", The char-

acteristic of biont that adapts to the survival envi-

ronment has emerged gradually through hundreds of

millions of years of evolution and optimization'P'!".

Practices also indicate that not the surface with more

uniform microstructure has more excellent property,but the property of material with nonuniform micro-

structure excels on the contrary. It has been found thatthe wear resistances of cast iron[5.6] and 3Cr2W8V

die steel[?] processed by laser are markedly im-

proved. The 3Cr2W8V is used as die steel. Wear

and fatigue are main failure mechanisms. How to

improve the properties and life-time of materials is

always one of the problems attracting material re-searchers with the developing of industry-'". The

surface of metallic material was processed by laserimitating the animals' body surface in this article,

which provides the fixed amount of experimentation

foundation and theories' basis for processing ad-vanced parts and die.

1 Experimental Procedure

The compositions of 3Cr2W8V die steel in theexperiment are shown in Table 1. The size of thespecimens used to wear testing was 6 mm long, 6 mm

wide and 30 mm high. The size of specimens used to

thermal fatigue testing was 40 mm long, 20 mm

wide and 3 mm thick, and a hole was drilled in one

side of every specimen so that the specimen could be

fixed onto the thermal fatigue test machine. The di-ameter of each hole is 3 mrn,

The striate surfaces on specimens were manu-factured using YAG laser imitating the striate shapeof dung beetle surface. The distributing spacing be-

Foundation Item, Item Sponsored by National Natural Science Foundation of China (50635030)

Bi~raphy, CHEN Li097I-). Female. Doctor. Associate Professor; E-mail: chli0423@I26.com: Revised Date: November 26. 2006

40 Journal of Iron and Steel Research. International Vol. 16

Table 1 Chemical composition of test steel %

C Si Mn Cr W v p S FeO. 35 O. 30 O. 40 2. 60 8. 02 O. 40 0.025 O. 025 Balance

tween bordered striations is 1. 2. 3. and 4 mrn , re-

spectively. The width and depth of unit are O. 9 and

0.3 mm , respectively. And specimens used to wear

testing and fatigue testing are shown in Fig. 1 and

Fig. 2. respectively. The laser current intensity is

200 A; the laser frequency is 10 Hz; and the laser

impulse duration is 3. 0 ms.

Micro-hardness of the material processed by laser is

measured on a type of HXD-lOOO micro-hardometer.

The distance between the tested two bordered dots

is about 0.2 mm and the load is 1 N. The used

method to test hardness is shown in Fig. 3 in detail.

The wear resistance of the materials is done at a pin-

on-disk abrasive wear tester. and the load is 1. 3. 5

and 8 N. respectively. The specimens are purged by

ultrasonic before and after the wear experiments.

then the mass loss is measured with a F A2004 cur-

rent analytical balance. and the mass losses of three

pieces of specimens at the same kind of laser pro-

cessing condition were averaged. The material worn

opposite with the specimens is carborundum sand

paper. The worn surface processed by laser is ob-

served by scanning electron microscope (SEM).

The thermal fatigue test machine was designed

to install multiple specimens simultaneously in order

to obtain specimens under identical testing conditions.

~

E/1 3mm

Fig. 1 Specimens with striate surface for wear testing

(a) With 2 mm spacing; (b) With 4 mm spacing

Fig. 2 Macrophotographs of striate surface for

thermal fatigue test

Fig. 3 Micrograph of processed zone and

hardness testing method

The specimens were heated by a resistance furnace

and cooled by running water. The furnace tempera-

tures throughout the test were monitored by a ther-

mocouple attached to the specimens at the center of

its length. The thermal fatigue test was carried out

in a temperature range between 700 'C and 25 C.

and the specimen was free from any applied load. A

complete thermal fatigue cycle included heating for

30 s up to the maximum cycling temperature (700 'C)

in resistance furnace and cooling for 10 s to the mini-

mum cycling temperature (25 'C) in water tank. For

every 200 cycles. the specimens were unloaded for

observing the characteristic. length and propagation

of crack on the specimens surface by optical micro-

scope and stereornicroscope , till the destined times

of cycles.

2 Results and Discussion

2. I Microstructure and microhardness of zone pro-cessed by laser

Fig. 3 is the microstructure of zone processed by

laser. The white and bright crescent zone is the

hardened zone in which the microstructure is very

fine [in Fig. 4 (a) J. and the black circle is the heataffect zone near the matrix material with coarsergrain [in Fig. 4 (b)].

Fig. 5 is the hardness curve of specimens with

different distributing spacing striations processed by

laser. With the spacing broadening. the micro-hard-

ness of the striations increases. As a result of the

distributing spacing being bigger. the area of the

matrix is bigger and the cooling rate is quicker rela-

Issue 4 Properties of 3Cr2W8V Die Steel With Striations Processed by Laser 41 ------------------ --------------------------------------------------------- -------------

( a) Processed zone; Cb) MatrixFig.4 Microstructure of material

8N:ON1 N

Mass loss/ I0' s 10, 2 II. 8 12, 2 18. 3

Table 3 Mass loss of specimens with 2 mm spacingstriations processed by laser under different load

Spacing I mm 2 mm :0 mm Unprocessed

Load

Table 2 Results of mass losses of specimens with differentspacing striations processed by laser4mm

200 L.- ~_~_~_~~_.....

1200

o 0.2 0.4 0.6 0.8 1.0Distance from edge of striation/mm

1.2 Mass loss/10-- 3 g 10.2 11. 8 12.2 18. :J

Fig. 5 Hardness curve of surfaces with differentspacing striations processed by laser

tively , so that the microstructure is finer and themicro-hardness of unit is higher. But the differenceis not very obvious as shown in upside of Fig. 5. In'addition, the hardness of processed zone is obviouslyhigher than that of the matrix material.

2.2 Wear resistanceTable 2 shows that the mass losses of speci-

mens with different spacing striations are less thanthat of unprocessed specimen, that is, the wear re-sistance is superior to that of unprocessed specimen.The wear resistance of material processed by laserimproves along with the decrease of the distributingdistance. When the spacing between bordered stria-tions is 1, 2, 3 mm , the wear resistance of the stri-ate specimen is 1. 79, 1. 55, and 1. 50 times of thatof unprocessed surface, respectively.

Table 3 is the mass loss of specimens with 2 mmspacing striations processed by laser under differentload. The mass loss increases with the increase ofload. When the load is 1. 3, 5 and 8 N, respectively,

the mass loss is 1, 1. 11, 1. 20 and 1. 72 times ofthat of the specimen loaded under 1 N. There is noobvious difference in the mass loss when the loadchanges below some value. When the load exceedssome value, the mass loss of material markedly in-creases. The results show that the zone processedby laser is a contribution to improve the wear resist-ance of material.

The force loading on the abrasive particle is di-vided to vertical and horizontal stress componentduring abrasive particle wearing. The first onemakes abrasive particles penetrate into the surface ofmaterial, and the later makes abrasive particles slideon the surface, so the cutting results in the furrow anddebris appearing on the surface seen from Fig. 6.

The abrasive particles slide on the horizontalsurface at the beginning of wearing. After wearing,some distance debris come being when the abrasiveparticles touch in softer matrix material. thus thedepth of surface shallows. However, the number offurrow and debris is all less, and the surface is higherthan the softer matrix, so the abrasive particlesmust get across a boss when the abrasive particles

42 Journal of Iron and Steel Research, International Vol. 16

Bouncing

Fig. 6 Process of abrasive particle wear

fall across the harder processed zone seen fromFig. 7. The vertical stress component (PI) is lessthan that (P) on the horizontal surface of matrixwhen the abrasive particles slide over the boss.i. e. the load decreases. At the same time, harderprocessed zone can make the moving of the abrasiveparticles turn from sliding to rolling over. so thecontacting area decreases. The projecting part staysbearing load and make the abrasive particles obtuse.The concave part gathers energy to protect matrixand adjust the stress status. Thus, the zone pro-cessed by laser plays a role in baffling and decreasingwearing, the mass loss decreased greatly, and as aresult. the wear resistance of the material with stri-ations processed by laser is improved to some ex-tent[7] .

2. 3 Thermal fatigue testAfter 4 000 thermal cycles of thermal fatigue, it

can be seen that on the surface of unprocessed speci-men, there are many thermal fatigue cracks withzigzag path and distributing in a cluster. However.on the surface of specimen with 4 mm spacing stria-tions, the crack is just a few in the number and rela-tively short in the length; and for the specimen with2 mm spacing striations. there is no crack on its sur-

PIP

Fig. 7 Pressure during wear process

face. There are fourth cracks on unprocessed speci-men in which the length of four cracks is more than5 mrn , and six is less than 1 mm. However. on thesurface of specimen with 4 mm spacing striation, thenumber of crack is only six which include one morethan 1 mm and three less than 1 mm. Evidently. thenumber of crack on the surface of unprocessed speci-men is much more than that of the specimens withstriations processed by laser. This indicates that thespecimens with striations have a superior thermal fa-tigue resistance compared to the unprocessed specimen.

The appearances of thermal fatigue crack propa-gation on the surface processed by laser are observedby optical microscope and stereornicroscope. It canbe seen that the main thermal fatigue crack isstopped (in Fig. 8) by the striation and bifurcated atthe striation. At the crack tip near the striation. thepath of crack propagation becomes more and morezigzag. It is obvious that the striations processed bylaser on the surface of steel have better thermal fa-tigue resistance and positive effect on inhibiting thecrack propagation. The crack must expend more en-ergy to propagate'f ' .

There is difference in microstructure betweenprocessed zone and matrix. The grains of micro-structure are finer after being processed by laser.and there is bigger dislocation density to increase thenumber of dislocation, intensity and toughness. Thestriations stud on the metal surface such as piles andnails. and they can retard the initiation and propaga-tion of crack. Thus, the fatigue strength of materialis markedly improved. The effect of striations on thematerial surface is similar to the "piles and nailseffect". The pile-nail effect can be described by acrack retardation mechanism and a stress counterac-tion mechanism.

Fig. 8 Appearance of fatigue crack propagation on thesurface processed by laser

Issue 4 Properties of 3Cr2W8V Die Steel With Striations Processed by Laser 43

3 Conclusions

(a) With bigger spacing; (b) With narrower spacing

Fig. 9 Model for crack retardation mechanism of pile-nail effect

(1) The striation with higher hardness would

lower load and sever the plough and pare off the ab-

rasive particles, and block the wear progress during

wear process, so the wear resistance is improved.(2) The wear resistance of material processed

by laser is better than that of unprocessed material.With the decrease of the distributing spacing between

Fig. 9 shows the model for crack retardation

mechanism of pile-nail effect. When the spacing be-

tween bordered two striations (pile-nail) is close,

with the relaxation of thermal stress and the con-

sumption of energy, the kinetics of crack propaga-

tion is much less than the resistance resulting from

the striation. As a result, the growth rate of crack

drops rapidly, even the propagation completely ter-minates at the striation, as shown in Fig. 9 (a).

When the spacing is far, the crack propagation is

held back by the striation firstly and the crack tip

near the striation becomes obtuse. Then, a new

crack nucleation is incubated opposite the striation

along the cycle direction. Afterwards, the maincrack and the crack nucleation grow toward the stri-

ation. Finally, the back and forth cracks are connected

by bridge of the striation, as shown in Fig. 9 (b). In

addition, because the striation was processed by la-

ser; the existing compressive residual stress on the

surface would counteract the remote tensile stressand cause the reduction of effective stress which controls

the thermal fatigue crack growth. It is easily to findthat because of the increase of thermal fatiguestrength and the reduction of effective stress on the

surface processed by laser, the effective stress islower than the thermal fatigue limit. As a result,

the thermal fatigue behavior of materials processed. by laser is markedly improved'Y'?".

References :

two bordered striations, the wear resistance of ma-

terial increases.

(3) The striations have a beneficial effect on

inhibiting thermal fatigue crack initiation and propa-

gation. The existing compressive residual stress on

the surface processed by laser would counteract the

remote tensile stress and cause the reduction of ef-fective stress which controls thermal fatigue crack

growth. The effective stress is lower than the ther-

mal fatigue limit. As a result, the thermal fatiguebehavior of materials processed by laser is markedly

improved.

(4) The "piles and nails effect" of striation is

the most important factor improving the wear resist-ance and thermal fatigue resistance of material.

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