Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in...

J. Mater. Sci. Technol., 2012, 28(3), 241–248.

Analytical Investigation of Prior Austenite Grain Size Dependence

of Low Temperature Toughness in Steel Weld Metal

X.F. Zhang1)†, P. Han2), H. Terasaki1), M. Sato2) and Y. Komizo1)

1) Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

2) Technical Development Department, Welding Business, Kobe Steel Ltd., 100-1, Miyamae, Fujisawa, Kanagawa 251-8551, Japan

[Manuscript received September 14, 2011, in revised form November 9, 2011]

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in thebainitic weld metals by in situ observations. Usually, decreasing the grain size is the only approach bywhich both the strength and the toughness of a steel are increased. However, low carbon bainitic steelwith small grain size shows a weakening of the low temperature impact toughness in this study. By directtracking of the morphological evolution during phase transformation, it is found that large austenite grainsize dominates the nucleation of intragranular acicular ferrite, whereas small austenite grain size leads tograin boundary nucleation of bainite. This kinetics information will contribute to meet the increasing lowtemperature toughness requirement of weld metals for the storage tanks and offshore structures.

KEY WORDS: In situ observation; Prior austenite grain size; Low temperature toughness;

Intragranular nucleation; Grain boundary nucleation

1. Introduction

Recently, the required low temperature toughnessof weld metals used for the storage tanks to transportthe crude oils and natural gases has become higherand higher due to extreme weather conditions and thestresses arising in these processes. For example, a liq-uefied natural gas storage tank (LNG storage) shouldhave the ability to store LNG at the very low temper-ature of –162 ◦C. Meanwhile, offshore structures haverecently been constructed with larger designs and op-erated in colder and deeper seas to explore energyresources. Usually, the brittle fracture is consideredmore dangerous than the ductile fracture in the lowtemperature. The rapid sinking of the Titanic is atypical example, because the low temperature brit-tle fracture made the hull steel and the wrought ironrivets failed[1]. Thus, improving the low temperaturetoughness of weld metal gets very important.

† Corresponding author. Ph.D.; Tel./Fax: +81 6 6879 4377;E-mail address: [email protected] (X.F. Zhang).

Grain refinement is an effective means for im-proving the strength and lowering the ductile-brittletransition temperature of structural alloys[2,3]. Gen-erally, the smaller the grain size, the stronger thematerial. In the case of the Hall–Petch effect, de-creasing the grain size can impede dislocation move-ment and increase yield strength; in the same time,it also decreased microcrack size and increased thetoughness[2]. It seems that decreasing the grain sizeis the only approach by which both the strength andthe toughness of a steel are increased. However, lowcarbon bainitic steel with small prior austenitic grainsize shows a weakening of the low temperature tough-ness in this study. It means that there are certainlyother factors contributing to the weakening effect dur-ing bainite transformation.

The aim of the present study is to address prioraustenite grain size dependence of the low tempera-ture toughness and the influence of the nucleation andgrowth mechanisms of bainite ferrite on the tough-ness. Therefore, the 780 MPa grade high tensilestrength low carbon bainitic steels were selected for

242 X.F. Zhang et al.: J. Mater. Sci. Technol., 2012, 28(3), 241–248.

Table 1 Chemical compositions of the investigated weld metals (in mass%) with balance Fe

Sample C Si Mn Ni Mo Al B N O

B11 0.04 0.28 1.77 2.54 0.74 0.004 0.0003 0.0039 0.0016

B22 0.03 0.34 1.86 2.53 0.78 0.004 0.0003 0.0042 0.007

B1 0.09 0.18 1.72 2.46 0.71 0.008 0.0006 0.0055 0.019

B2 0.07 0.24 1.57 2.38 0.71 0.008 0.0006 0.0055 0.030

B3 0.10 0.17 1.26 2.49 0.73 0.008 0.0005 0.0055 0.046

M1 0.11 0.92 1.73 2.37 0.72 0.068 0.0026 0.0054 0.010

M2 0.10 0.80 1.76 2.49 0.73 0.046 0.0022 0.0056 0.014

M3 0.09 0.62 1.75 2.50 0.73 0.036 0.0017 0.0055 0.015

Fig. 1 (a) Schematic illustration of optical system in LSCM, (b) thermal cycle applied to the weld metals

investigation. For a reference, the martensitic steelswere also taken into account. To better understand it,an advanced observation system, which combines aninfrared image furnace with laser scanning confocalmicroscopy (LSCM), was developed to direct track-ing of the morphological evolution at the micrometerscale under a rapid heating and cooling cycle. Thistechnique has made it possible to identify microstruc-tural changes at any temperature in real time. Finally,the results were compared and analyzed with previ-ous reports in order to provide further insight intothe correlation of grain size with the low temperaturetoughness.

2. Experimental

The test materials were low carbon submerged arcweld (SAW) metals with varied oxygen content. Inorder to obtain the weld metals with less oxygen con-tent, gas tungsten arc welding (GTAW) method wasalso used in this study. The chemical compositions ofthe weld metals used in this investigation are shownin Table 1. The SAW samples of B1, B2, and B3represent the bainitic weld metals, while those of M1,M2, and M3 characterize the martensitic weld met-als. The GTAW weld metals with bainitic microstruc-ture were marked as B11 and B22. The objectiveswere to determine how oxygen content might affectthe prior austenite grain size and the low tempera-ture toughness. LSCM system was used to in situobserve morphological development of grain growthand phase transformation, as shown in Fig. 1(a). De-

tails of this system have been described elsewhere[4,5].The specimen was machined into 5 mm in diameterand 1 mm in height, and the observed plane was mir-ror polished. Then, this disc sample was set in theinfrared image furnace and continuously heating andcooling along a heat history, as displayed in Fig. 1(b).The atmospherics was filled with high purity argonshielding gas after evacuating to 1×10−2 Pa. TheLSCM images were recorded in the time resolution of0.03 s throughout the thermal cycle.

The samples were also submitted to the CharpyV-notch impact test for impact toughness analysis.The test temperature was –60 ◦C. Furthermore, to de-tect the chemical compositions and particle diameterdistribution of the inclusions, electron probe micro-analyzer with energy dispersive X-ray spectroscopy(EPMA-EDX) was used in this study.

3. Results and Discussion

Fig. 2 displays the relationship among the oxy-gen content, prior austenite grain size, and absorbedenergy. More than one samples were carried out forimpact test, and the detailed absorbed energy dataare displayed in Table 2. The average absorbed en-ergy data were used in Fig. 2. For martensitic weldmetals, the prior austenite grain size decreases withincreasing oxygen content, whereas the absorbed en-ergy increases. It means that increasing the oxygencontent can inhibit the austenite grain growth and im-prove the low temperature toughness. It also suggeststhat decreasing the grain size can decrease microcrack

X.F. Zhang et al.: J. Mater. Sci. Technol., 2012, 28(3), 241–248. 243

Table 2 Detailed absorbed energy data, oxygen content, and grain size of theinvestigated weld metals

Sample Absorbed energy Oxygen content Grain size

at −60 ◦C/J /×10−6 /μm

B11 318 316 301 16 198

B22 227 222 187 70 150

B1 128 121 115 190 112.5

B2 111 104 101 300 100

B3 70 67 65 460 90

M1 58 53 52 100 110

M2 60 59 53 140 90

M3 70 66 64 150 85

Fig. 2 Relationship among the oxygen content, prioraustenite grain size, and absorbed energy in theweld metals

size and increase the impact toughness. This satisfiedthe knowledge described in the section 1. However,decreasing the austenite grain size does not guaranteegood low temperature toughness of the bainitic weldmetals. As displayed in Fig. 2, both the austenitegrain size and absorbed energy decrease with increas-ing oxygen content in the bainitic weld metals. Thisindicates that increasing the oxygen content can alsoinhibit the austenite grain growth for bainitic weldmetals, but reduce the low temperature toughness.Thus, the larger the grain size, the better the tough-ness. This is clearly contrary to the knowledge de-scribed in the section 1. In a word, decreasing austen-ite grain size can improve low temperature toughnessin martensitic weld metals, while the reverse effectpresents in the bainitic weld metals.

To understand the above paradox, one can firstexplore the effect of the oxygen content on the graingrowth. Fig. 3 shows the chemical compositions ofthe inclusions in bainitic weld metals and their parti-cle diameter distribution. The equivalent diameter ofthe inclusions was determined by EPMA-EDX analy-sis. The letter N represents the number of inclusionswhose size is more than 0.3 μm. The number of in-clusions was counted in a field of view of 96800 μm2.For B1 weld metal, the number of the inclusions isabout 448 and the oxygen content is 190×10−6. EDXanalysis shows that the inclusions are mainly Al–O,Mn–O, Si–O and Al–N, as shown in Fig. 3(a). For

B2 weld metal, the number of the inclusions is about632 and the oxygen content is 300×10−6. The inclu-sions are distinguished to be Al–O, Mn–O, Si–O andAl–N, as shown in Fig. 3(b). In the case of B3 weldmetal, the inclusions are similar to those of B1 and B2weld metals. But the number of the inclusions in B3weld metal is increased to 746 with 460×10−6 oxygencontent, as shown in Fig. 3(c). Based on the above,the number of the inclusions increases with increas-ing oxygen content. To make it clear, the relation-ship between equivalent particle diameter and the in-clusions number density is characterized in Fig. 3(d).The largest number of the inclusions is presented inB3 weld metal, while the least is contained in B1weld metal. As the Zener pinning effect described,the grain boundary migration can be retarded by thepresence of particles[3,6]. Small particles act to pre-vent the motion of such boundaries by exerting apinning pressure which counteracts the driving forcepushing the boundaries. The second phase particles,such as non-metallic inclusions, have a strong pinningeffect on grain boundary, thereby inhibiting the graingrowth[3,6]. In a word, the greater the number of theinclusions, the stronger the inclusions on the grainboundary pinning effect, so the crystal grain size getsrefined. Thus, the prior austenite grain size decreaseswith increasing oxygen content. This is also appliedto the martensitic weld metals.

Since low carbon bainitic steel with small grainsize shows a weakening of the low temperature im-pact toughness as shown in Fig. 2, it means that thereare certainly other factors contributing to the weak-ening effect during bainite transformation. Fig. 4 dis-plays in situ morphological evolution at the microm-eter scale under the cooling cycle. Fig. 4(a)–(d) showthe bainite transformation of B1 weld metal duringthe cooling process, while Fig. 4(e)–(h) display thetransformation of B3 weld metal. In the case of B1metal, the nucleation site of bainite can be dividedinto two categories, one is grain boundary and inclu-sions where primary bainitic ferrite nucleates, and theother is surface of the primary bainitic ferrite wheresympathetic nucleation occurs. In Fig. 4(a) and (b),the bainitic ferrite first takes place at the inclusionand grain boundary, but the main nucleation sites areat the inclusions, as designed by the arrows. Subse-


Fig. 3 Chemical compositions of the inclusions in bainitic weld metals and their particle diameter distribution.The letter N represents the number of inclusions whose size is more than 0.3 μm. The number of inclusionswas counted in a field of view of 96800 μm2. (a) B1 weld metal, (b) B2 weld metal, (c) B3 weld metal,(d) relationship between equivalent particle diameter and the inclusions number density

quently, the secondary bainitic ferrites nucleate sym-pathetically on the surface of the primary bainitic fer-rite, as indicated in Fig. 4(c). With the growth ofbainitic ferrites, impingement events take place be-tween the ferrite plates. In fact, this interlockingnature and the impingement of the bainitic ferritescan be clearly observed in Fig. 4(d). Usually, thisintragranular bainitic ferrite is called acicular ferrite.Acicular ferrite is formed in the interior of the origi-nal austenitic grains by direct nucleation from the in-clusions, resulting in randomly oriented short ferriteneedles with a basket weave appearance. This inter-locking nature, together with its fine grain size, pro-vides the maximum resistance to crack propagation bycleavage. So, this microstructure is advantageous overother microstructures because of its chaotic ordering,which increases toughness[7–9]. Thus, the low tem-perature toughness of B1 weld metal is good. Basedon in situ observations, it seems that intragranulartransformation tends to occur in the large grains.

For B3 weld metal with small grain size, the nu-cleation mechanism is different from that of B1 weld

metal with large grain size. This can be distinguishedin Fig. 4(e)–(h). In Fig. 4(e) and (f), the bainiticferrite first takes place at the grain boundary. Al-most all of the bainitic ferrites nucleate at the grainboundaries, and only a ferrite nucleates at the inclu-sion, as shown in Fig. 4(g). Usually, the bainitic fer-rite nucleated at austenite grain boundary is calledgrain boundary ferrite. From Fig. 4(g) and (h), it canbe seen that the grain boundary ferrites grow in thesame direction parallel to each other. Although thegrowth of the grain boundary ferrite can also causeimpingement between the plates, the microstructureseems more orderly (Fig. 4(g) and (h)). Comparedto acicular ferrite, the grain boundary ferrite has asmaller contribution to the low temperature tough-ness. Thus, the toughness of B3 weld metal is not sogood as that of B1 weld metal.

To better grasp the correlation of prior austenitegrain size with the nucleation of bainite, B11 samplewith large grain size was selected for study. Mean-while, B1 sample with slightly larger scale inclusionswas also re-examined to obtain the typical acicular


Fig. 4 In situ observations of the morphological evolution and microstructural changes of bainitic ferrite.(a)–(d) snapshots of the LSCM images for B1 weld metal, (e)–(h) snapshots for B3 weld metal


Fig. 5 In situ observations of the intragranular transformation evolution of bainitic ferrite. (a)–(e) snapshots ofthe LSCM images for B11 weld steel with large austenite grain size, (f)–(j) snapshots for B1 weld metalwith slightly larger scale inclusions


ferrite structure. Fig. 5(a)–(e) display the microstruc-ture evolution of B11 sample during cooling, whileFig. 5(f)–(j) show the typical acicular ferrite forma-tion in the sample of B1. For B11 sample with largeaustenite grain size, a clear polygonal grain boundarycan be observed in Fig. 5(a). With decreasing tem-perature, intragranular bainite nucleation and growthoccur (Fig. 5(b)–(d)), and then impingement eventstake place between the ferrite plates with the growthof bainitic ferrites (Fig. 5(e)). In the case of B1 sam-ple with slightly larger scale inclusions, the potentialnucleation sites are marked in Fig. 5(f). Subsequently,the acicular ferrites nucleate on the potential sites(that is, inclusions), as shown in Fig. 5(g)–(i). Fi-nally, the randomly oriented short ferrite needles witha basket weave appearance are presented in Fig. 5(j).

In addition, when the austenite grain size is large,the number density of inclusions becomes large rela-tive to boundary nucleation sites promoting the for-mation of acicular ferrite at the expense of bainite.Similarly, a small grain size has a relatively largenumber density of grain boundary nucleation sites sobainite dominates the microstructure. Therefore, al-though the grain sizes of B1 and B11 weld metals withsmall number of inclusions are large, their toughnessare better than that of B3 weld metal. These obser-vations are consistent with the previous results[10].

Oxides in steel weld metals can initiate fractureby influencing the microstructures. For example, thenon-metallic particles can initiate brittle cracks orvoids during ductile failure, both events leading toa deterioration in mechanical properties[11,12]. In thesame time, the non-metallic inclusions also contributeto the formation of acicular ferrite, which improvesthe toughness[10,13]. Thus, changes in toughness withoxygen concentrations also need be highlighted ex-cept for prior austenite grain size dependence of lowtemperature toughness. In this study, excess oxygencontent (from 16×10−6 to 460×10−6) prefers to de-crease the toughness for 780 MPa grade high strengthbainitic weld metals, while increasing oxygen contentin martensitic weld metals contributes to improve thetoughness. Terashima and Bhadeshia[14] reviewed theinvestigation of oxygen content on the toughness, andthey pointed out that the absorbed energy decreasesup to 100×10−6 oxygen content and then peaks occurat 300×10−6 in the high strength alloy (778 MPa).This non-monotonic change of the toughness is dueto the microstructure changes with varied oxygen con-tent. They considered that the microstructure is dom-inated by grain boundary nucleating upper and lowerbainite, respectively, along with martensite. Withincreasing oxygen content, the acicular ferrite is ab-sent from the microstructure, so that oxides simplyserve to nucleate fracture. But for this study, the mi-crostructures of B1, B2, B3, B11, and B22 are bainitebased on in situ observations. Although the intra-granular acicular ferrite is also absent from the mi-crostructure with increasing oxygen content, the oxide

particles mainly serve to form acicular ferrite not tonucleate fracture. In Figs. 4(a)–(d) and 5, intragran-ular nucleation is the main phase transition mecha-nism in the weld metals with 190×10−6 and 16×10−6

oxygen content, respectively. Although grain bound-ary nucleation is main transition mechanism in theweld metal with 460×10−6 oxygen content (Fig. 4(e)–(h)), intragranular nucleation still appears. Thus, thetoughness improves monotonically as the oxygen con-centration is reduced, which is different from the pre-vious study by Terashima and Bhadeshia[14]. By con-trast, it can be seen that the changes in grain sizecaused by varied oxygen content lead to the differ-ence of the low temperature toughness in bainitic weldmetals.

Above mentioned show that intragranular trans-formation tends to occur in the large grains, that is,the larger the grain, the easier the acicular ferrite gen-eration. And the formation of acicular ferrites willgreatly contribute to increase the toughness. As re-ported, the nucleation of intragranular formation acic-ular ferrite is aided by non-metallic inclusion, in par-ticular oxygen-rich inclusions[10]. In this study, theinclusions in B1, B2, B3, B11, and B22 are distin-guished to be Al–O, Mn–O, Si–O and Al–N, whichmust be responsible for the nucleation of acicular fer-rite. In general, compared to the grain refinement, thespecial microstructure is more important for the im-provement of the toughness. Therefore, although thegrain sizes of B1 and B11 weld metals are larger thanthat of B3 weld metal, the low temperature tough-ness of B1 and B11 is better than that of B3. Thus,based on the experimental observations, intragranularacicular ferrite is regarded as the most desirable mi-crostructure feature than the grain boundary ferrite,in view of its strength and the low temperature tough-ness. Finally, appropriate increase in the austenitegrain sizes by composition control, which enhancesthe low temperature toughness of bainitic steels, isextremely important in engineering application.

4. Conclusion

In conclusion, the relationship among the oxy-gen content, prior austenite grain size, and absorbedenergy were investigated in steel weld metals. Formartensitic weld metal, the austenite grain size de-creased with increasing oxygen content, but the lowtemperature toughness increased. It conformed to theHall–Petch effect. But for the bainitic weld metal,both austenite grain size and the low temperaturetoughness decreased with increasing oxygen content.It was ascribed to large austenite grain size dom-inating the nucleation of intragranular acicular fer-rite, whereas small austenite grain size leads to grainboundary nucleation of bainite.

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