J. Mater. Sci. Technol., Vol.23 No.5, 2007 659

Effect of Microstructure Refinement on the Strength and Toughness

of Low Alloy Martensitic Steel

Chunfang WANG†, Maoqiu WANG, Jie SHI, Weijun HUI and Han DONGNational Engineering Research Center of Advanced Steel Technology, Central Iron and Steel Research Institute, Beijing

100081, China

[Manuscript received October 16, 2006, in revised form December 25, 2006]

Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain sizeranging from 6 µm to 199 µm has been characterized by optical metallography (OM), scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM). The yield strength and the toughness of thesteel with various prior austenite grain sizes were tested and correlated with microstructure characteristics.Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steelfollow a Hall-Petch relation with the yield strength. When the prior austenite grain size was refined from199 µm to 6 µm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at77 K improved more than 8 times, indicating that microstructure refinement is more effective in improvingthe resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unitcrack path for cleavage fracture is identified as being the packet.

KEY WORDS: Martensitic steel; Grain refinement; Strength; Impact toughness; Cleavage fracture

1. Introduction

The lath martensite structure is one of the mostimportant structures in steels since commercial, heat-treated low alloy structural steels usually have lathtype martensitic structures. The lath type marten-sitic structure is composed of packets of parallel lathswithin the prior austenite grain[1,2]. A prior austenitegrain is divided into several packets.

It is well known that strength and toughness ofmartensitic steels improve as the grain size is refined.Because of complicated microstructures, the effectivegrain size to the strength and toughness is not en-tirely clear for martensitic steels[3–6]. Many investi-gators have shown that the prior austenite grain sizeplays a role in controlling the strength and tough-ness of steels having lath martensite. For example, aHall-Petch type relationship was observed to exist be-tween the prior austenite grain size and the strengthof several steels[7–12]. However, some other investi-gators regarded that the yield strength and tough-ness are dependent on the packet size, but not on theaustenite grain size[13–16]. Naylor once claimed thatthe lath width is the basic parameter controlling theyield stress and the ductile-to-brittle transition tem-perature of Fe-Mn and Fe-Mn-Cr low carbon steelswith lath martensite or baintic structures[17].

The above survey of literature show the complexof microstructure on the mechanical properties of lathmartensitic steels. Therefore, the purpose of the in-vestigation is to examine the effect of microstructurerefinement on the strength and toughness of low car-bon martensitic steels.

2. Experimental

A commercial 17CrNiMo6 steel was used in

† Ph.D. Candidate, to whom correspondence should be ad-dressed, E-mail: fang2004@126.com.

this investigation, with the chemical composition inwt pct as follows: C 0.17, Si 0.21, Mn 0.55, P 0.011,S 0.002, Cr 1.80, Ni 1.58, Mo 0.25. The steel wasprovided as hot-rolled round bars with a diameterof 70 mm. These round bars were further reheatedand forged into round bars with a smaller diameterof 16 mm, followed by annealing to facilitate ma-chining. Tensile specimens with a diameter of 5 mmand Charpy U-notch specimens with dimensions of10 mm×10 mm×55 mm were machined from theround bars.

The specimens were austenitized at 1133–1473 Kfor 1 h in air furnace and then quenched into wa-ter, followed by tempering at 443 K for 2 h. Themean prior austenite grain sizes of the specimens thatwere austenitized at 1133, 1213, 1323, 1373, 1423 and1473 K were 11, 17, 26, 50, 129 and 199 µm, respec-tively. Cyclic heat treatment of the specimens wasalso carried out at 1133 K for 4 cycles in salt bath fur-nace, resulting in a fine austenite grain size of 6 µm.

The tensile test was performed on a WE-300 ten-sile machine at room temperature (298 K) at a strainrate of 2.5×10−3–2.5×10−4/s in accordance with thestandard GB/T 228-2002. The impact test was per-formed on a JBN-300B impact machine with a ham-mer velocity of 5.5 m/s at 77 K in accordance withthe standard GB/T229-1994.

The microstructure was characterized by opticalmetallography (OM), scanning electron microscopy(SEM), and transmission electron microscopy (TEM).A HITACHI S-4300 type scanning electron micro-scope and a HITACHI H-800 type transmission elec-tron microscope were used. To reveal the prioraustenite grains, the specimens were etched with asupersaturated picric acid aqueous solution after me-chanical polishing. The prior austenite grain sizes inthe 17CrNiMo6 steel were determined by linear in-tercept measurements on optical micrographs. Then,the specimens were etched with a 2% nital to delineatethe lath microstructure within the prior austenite

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Fig.1 Optical micrographs (a) and (c) showing the austenite grains and the corresponding SEM micrographs (b)and (d) showing martensitic packets in (a) and (c) with the prior austenite grain sizes of 50 and 11 µm,respectively

Fig.2 TEM bright field images showing martensitic laths in the 17CrNiMo6 steel: (a) austenized at 1473 K and(b) cyclic heat treated at 1133 K

grains. Packet sizes were measured by linear interceptmeasurements on SEM images. Fracture surfaces ofthe impact specimens were observed on a scanningelectron microscope. Thin-foil specimens for TEM ob-servations were cut from the specimens and preparedby twin-jet electrolytic polishing using a solution of10% perchloric acid and 90% ethanol at 243 K.

3. Results and Discussion

3.1 Microstructure characterization

Figure 1 shows the optical and SEM micrographsof the specimens with the prior austenite grain sizes of50 µm (Fig.1(a) and (b)) and 11 µm (Fig.1(c) and (d))for the 17CrNiMo6 steel. In the steel, packets weredistinctly present within the prior austenite grains, as

shown in Fig.1(b) and (d). The laths formed within aprior austenite grain grouped themselves into severalpackets. The martensitic packet size decreased withthe refinement of the prior austenite grains.

Figure 2 shows TEM bright field images of thespecimens which were austenized at 1473 K (Fig.2(a))and cyclic heat treated at 1133 K (Fig.2(b)) to obtainaustenite grain size of 199 and 6 µm, respectively.Dislocations were observed within the laths of bothspecimens. From TEM observations, the lath widthwas estimated to be about 0.3 µm, which was almostindependent of the prior austenite grain size, as re-ported by Roberts[7].

Figure 3 shows the packet size distribution frac-tion for the specimens with the prior austenite grainsizes of 6, 17, 26 and 199 µm, respectively. About 200

J. Mater. Sci. Technol., Vol.23 No.5, 2007 661

Fig.3 Distribution of martensite packet size in 17CrNiMo6 steel with the prior austenite grain sizes: (a) 6 µm,(b) 17 µm, (c) 26 µm and (d) 199 µm

Fig.4 Relationship between the packet size and the prioraustenite grain size in the 17CrNiMo6 steel

Fig.5 True stress vs true strain curves for the specimenswith the prior austenite grain sizes of 199, 50 and6 µm

packets in each specimen were measured from theSEM images and the average packet size was thenobtained. Figure 4 shows the relation between thepacket size (dp) and the prior austenite grain size(dγ). The martensite packet size decreased with therefinement of the prior austenite size, with the averagemartensite packet size decreased from 109 µm to 4 µmas the austenite grain size decreased from 199 µm to6 µm.

3.2 Influence of microstructure refinement on theyield strength

Figure 5 shows the true stress vs true strain curvesfor the specimens with the prior austenite grain sizesof 199, 50 and 6 µm. It can be seen that the totalelongation increased with the refinement of the prioraustenite grains.

Figure 6 shows the Hall-Petch type plots of theyield strength as a function of the prior austenite size(Fig.6(a)) and the packet size (Fig.6(b)), respectively.Generally the yield strength of metals is given by theclassic Hall-Petch relation:

σy = σ0 + kyd−1/2 (1)

where σ0 and ky are constants with ky being the Hall-Petch slope, and d is the mean effective grain size.It can be seen from Fig.6 that a linear relationshipexisted between the yield strength (Rp0.2) and thereciprocal of the square root of the prior austenitegrain size (d−1/2

γ ) as well as between the yield strengthand the reciprocal of the square root of the packetsize (d−1/2

p ), indicating the dependence of the yieldstrength of the 17CrNiMo6 martensitic steel on theprior austenite size and on the packet size.

662 J. Mater. Sci. Technol., Vol.23 No.5, 2007

Fig.6 Dependence of the yield strength on (a) the prior austenite grain size and (b) the martensite packet sizefor the 17CrNiMo6 steel

Fig.7 Effect of (a) the prior austenite grain size and (b) the martensite packet size on the impact energy AKU2

at 77 K for the 17CrNiMo6 steel

Fig.8 Fracture surfaces of the impact specimens tested at 77 K for the prior austenite grain sizes of (a) 6 µm,(b) 17 µm, (c) 26 µm and (d) 199 µm

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Table 1 Relationship between the grain size and packet size andarithmetic mean of the cleavage facet size for the 17CrN-iMo6 steel

Grain size, dγ/µm Packet size, dp/µm Cleavage facet size/µm199 109 96129 31 2650 19 1726 12 1317 9 911 7 76 4 5

Fig.9 Profile fractograph of the fractured impact speci-men in the 17CrNiMo6 steel with the prior austen-ite grain size of 199 µm

In a previous study of Fe-0.20C-Ni-Cr-Mo steel,Tomita regarded the packet size as the effective grainsize for the lath martensite from the results thatthere was a Hall-Petch relationship between the yieldstrength and the packet size, but no such relationshipbetween the yield strength and the prior austenitegrain size[15]. The present results showed that it wasdifficult to tell that for the 17CrNiMo6 steel whetherthe packet size or the prior austenite grain size wasthe effective grain size simply from the Hall-Petch plotsince both sizes had a Hall-Petch relationship with theyield strength.

As the prior austenite grain size in the 17CrN-iMo6 steel was refined from 199 µm to 6 µm, the yieldstrength increased by 235 MPa, being only a 25% in-crease, indicating that the grain refinement was notvery effective in increasing the strength. The data forthe Fe-0.2C martensitic steel[13] and the Fe-0.20C-Ni-Cr-Mo martensitic steel[15] are also included in Fig.6.There was good agreement between the results forthe 17CrNiMo6 steel and for the Fe-0.20C-Ni-Cr-Mosteel, which was presumable due to similar carbonand alloy contents in the two steels. The dependenceof the yield strength on the packet size was not sostrong for the two low alloy steels, as indicated bythe low slope of the Hall-Petch plot, although the Fe-0.2C steel showed weaker dependence. Accordingly,the microstructure refinement was not so effective inincreasing the strength for lath martensitic steels.

3.3 Influence of microstructure refinement on the im-pact toughness

Figure 7 shows the relationship between the prioraustenite grain size and the Charpy impact energy(AKU2) at 77 K for the 17CrNiMo6 steel. The impact

energy value increased with decreasing prior austen-ite grain size and it increased more than 8 times from5.7 J to 46.7 J when the prior austenite grain sizewas refined from 199 µm to 6 µm, indicating thatgrain refinement contributed largely to the toughnessof martensitic steels.

Figure 8 shows the fracture surfaces of the impactspecimens with the prior austenite grain sizes of 6, 17,26 and 199 µm. The fractured surfaces of the 17CrN-iMo6 steel consisted of quasi-cleavage facets, and com-plex river patterns consisting of small cleavage steps,were also observed within the facets. In this case,a quasi-cleavage facet was defined as the spacing di-vided by heavy tear lines. The arithmetic mean ofthe observed facet size was determined by linear in-tercept method on SEM images, and the results isgiven in Table 1. For the specimens with the prioraustenite grain sizes ranging from 199 µm to 6 µm,the arithmetic mean of the quasi-cleavage facet sizewas decreased from 96 µm to 5 µm, which was cor-responding to the measured values of the martensitepacket size of from 109 µm to 4 µm, respectively. Itimplied that the dominant microstructural feature inthe toughness of the martensite was the packet size.

Further evidence of the direct relationship betweenthe packet size and the facet size is shown in Fig.9,where the broken impact specimen with the prioraustenite grain size of 199 µm was observed along thelongitudinal direction after it was cut, polished andetched by 2% nital. The cleavage crack usually largelychanged its direction at the packet boundaries, due tothe fact that a local cleavage cracking event across alarge packet could lead to an instant load drop, butwhen the crack attempted to run from one packetto another, the crystallographic orientation and mi-crostructure changed, so that more work had to bedone to cross the packet boundary. As a result, thelocal cleavage crack might be arrested.

From the above results, it follows that grain refine-ment was very effective in improving the resistance tothe cleavage fracture, but was relatively ineffective inincreasing the strength.

The basis on the classic “pile-up” model of theHall-Petch relation gives,

ky ∼ 3[2Gbτb

qπ

]1/2

(2)

where q is a geometric factor of order unity; G is theshear modulus; b is the Burgers vector; and τb is thecritical shear stress for the transmission of slip acrossa grain boundary. The Burgers vector is fixed by thecrystal and the shear modulus can only be changedby significantly changing the composition of the steel.

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The controllability of the Hall-Petch coefficient in thisequation is the variable parameter τb. This parame-ter is affected by the nature of the grain boundary.In iron, the primary slip planes are the 110 planes. Itis expected to increase with the misorientation of thedominant <110> slip planes. Increasing the typicalmisorientation should increase τb, hence ky, while de-creasing misorientation should decrease ky. To max-imize the strengthening that can be accomplished bygrain refinement, ky should be as large as possible.

For iron cleaves on {100} planes, as has beenproved[18,19], the appropriate measure of the grainsize for cleavage should be the coherence length on{100}. Toughness should increase with the misorien-taion of {100} planes across the boundaries. Sincethe lath boundaries tend to lie parallel to {110}, thelaths share common {100} planes that cut throughthe boundaries. In present study it shows that im-pact energy increased more than 8 times and theyield strength increased only 25%. So transformationfor refining the grain size in present study benefitedagainst cleavage as the misorientation along {100}planes was significant, however it might have onlyslight misorientation of the [110] slip planes. As aresult, grain refinement was very effective in improv-ing the resistance to the cleavage fracture, but wasrelatively ineffective in increasing the strength.

4. Conclusions

(1) There is a Hall-Petch relationship between theyield strength and the prior austenite grain size andthe packet size for the 17CrNiMo6 steel. The strengthand impact toughness of the 17CrNiMo6 steel can beimproved with the refinement in the prior austenitesize and the packet size.

(2) Microstructure refinement is more effective inimproving the resistance to the cleavage fracture thanin increasing the strength. The prior austenite grainsize in the 17CrNiMo6 steel was refined from 199 µmto 6 µm, the yield strength increased by 235 MPa, be-ing only a 25% increase and the impact energy valueincreased more than 8 times.

(3) The cleavage crack usually largely changed itsdirection at the packet boundaries. The unit crackpath for cleavage fracture was identified as being thepacket.

AcknowledgementsThe authors would like to thank Mr. Wenhan Zhang

for help in the experiment of revealing the prior austen-ite grain size. This work was supported by the GeneralArmaments Department Beforehand Research Foundation(No. 9140A12050306QT0901).

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