The characteristics of secondary hardening and softening during tempering in 3%Cr-1%Mo hot work tool steel.
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The quenched and tempered hardness fluctuates with microstructures as quenched. The effect of quenched microstructures such as martensite, bainite and retained austenite on the tempering characteristics was studied, especially on secondary hardening and softening during tempering in 3% Cr-1% Mo hot work tool steel.As a result, it was confirmed that the tempering characteristics, such as hardness, microstructures and sequence of changes in the carbide structures were significantly affected by retained austenite. The hardness after tempering above 500°C increased with retained austenite. The increment of secondary hardening, H in Rockwell C scale, is directly proportional to the percentage of volume fraction of retained austenite in as-quenched condition, Vγ, following the equation: H=0.33Vγ. The resistance of softening on tempering above 600°C also increased with increased retained austenite.It was assumed that the rate determining process of secondary hardning is the diffusion process of carbon in austenite and that of softening during tempering is the diffusion process of chromium in ferrite from their activation energies.Keywords:
Tempering
Hardening (computing)
Work hardening
The time required for carbon diffusion from martensite to enrich surrounding austenite from 0.27 to 1.04% C during the formation of low-carbon matensite is 10~(-7)s in order of magnitude as calculated. It does prove that the diffusion of carbon atoms can keep pace with the formation of lath martensite. From thermodynamical calculation, it is reasonable to recognize that the precipitation of carbon from martensite results the enrichment of austenite. TEM observation revealed that the quenched structure in a 0.12C-low Ni-Cr steel mainly contains lath martensite and interlath retained austenite, and also twin martensite. The existence of the latter further confirms the occurrence of carbon diffusion to enrich austenite during the martensite formation, and twin martensite forms at the parent phase where carbon enrichment is not very high. The interface of austenite and martensite is somewhat straight. The typical upper bainite (B_Ⅱ), B_Ⅲ type bainite and carbide-free bainite (B_1) can aU appear in the ame steel and there exists superledges at the interface of austenite and bainitie ferrite which is quite different to that of austenite and martensite. In addition, from the kinetics point of view. the growth rate of low-carbon martensite is 3 to 4 order of magnitude greater than that of upper bainite. It is more likely to conclude that the mechanism of the formation of lath martensite is not identical with that of bainite.
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Atomic force microscopy was employed to quantitatively study the surface relief accompanying martensite and bainite in a Cu–Zn–Al alloy. It is demonstrated that the surface relief angle associated with martensite is 14.3°, in good agreement with the theoretical result deduced from the phenomenal theory of martensite crystallography (PTMC). However, the surface relief angle associated with bainite is 2.0°–3.2°, which disagrees with the PTMC result. This indicates that the transformation mechanism of bainite is different from that of martensite. The fine structures of the surface relief associated with martensite and bainite are also investigated. The surface relief of martensite is composed of the small parallel relief caused by small martensite plates, and that of bainite is composed of small cells induced by subunits in bainite.
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During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters.
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Abstract Self-tempering effects can be observed in steels with relatively high martensite start temperatures. After the formation of the first martensitic laths, carbon is able to diffuse in these laths during cooling, which can be attributed to sufficiently high temperatures. This effect cannot be observed in laths formed at lower temperatures. In steels containing up to 0.2 m.-% carbon, up to 90 % of the carbon atoms in the martensite segregate to dislocations during quenching. Due to its atomic resolution and sensitivity with respect to light elements, atom probe tomography is very well suited for the investigation of this phenomenon. In this study, the self-tempering effect in a quenched and tempered steel 42CrMo4 with a martensite start temperature of 310 °C is investigated by means of atom probe tomography.
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The kinds of carbides and the sequence of their precipitation in lower bainite transformation of high carbon steel GCr15 have been investigated.It is identified that there exist all kinds of carbides similar to those of high carbon tempering martensite,that is,the appearance of carbides η-Fe_2C,χ-Fe_5C_2 were discovered in lower bainite transformation and the carbides could precipitate in two kinds of orders : the first in complicated process of η(e)→χ→θ and the second in simple way of e→θ. It could be the result of pre-transformation and carbon segregation in matrix. In conclusion both transformation of lower bainite and tempering martensite in high carbon steels have the general character of shear and supersaturation,but the phenomenon of pre-transformation appears only in the former.
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두 種類의 martensite合金, 즉 Fe-C基 martensite鋼(Fe-1.7%C)과 Fe-Ni基 martensite合金(Fe-27% Ni-0.14%C)을 마련해서, 이 두 martensite 조직 중에 含有된 잔류 austenite의 tempering 擧動을 X-線的으로 調査하여 다음과 같은 結論을 얻었다.
1. Fe-1.7%C martensite 鋼의 잔류 austenite는 約 150℃×1hr tempering에서 分解하기 시작하였으며 280℃×1hr tempering에서는 거의 大部分 分解하였다.
2. Fe-1.7%C martensite 鋼中의 殘留 austenite의 (111)γ 回折線의 積分幅은 tempering 溫度와 더불어 增加하였다.
3. Fe-27% Ni-0.14%C martensite 鋼은 430℃까지 tempering하여도 殘留 austenite가 分解하지 않았다.
4. Fe-27% Ni-0.14%C martensite 鋼에 있어서 austenite의 積分幅은 360℃ tempering 까지는 거의 一定한 값을 나타내다가 그 以上 溫度가 增加함에 따라 減少하였다.
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AbstractAbstractRecently published experimental data demonstrate that the strength of mixed microstructures of tempered bainite and martensite can peak at an intermediate volume fraction of martensite. In the present work, a quantitative interpretation of these observations is achieved by modelling the mechanical properties of bainite and martensite in their tempered states. It is found that the peak in the curve of the strength as a function of the volume fraction of martensite can be attributed to two factors. When bainite forms it enriches the residual austenite with carbon, so that the strength of the subsequent martensite increases. In addition, during its deformation, the strength of the bainite is enhanced via plastic constraint by the surrounding stronger martensite. Taking these effects into account, it is possible to predict accurately both the trends and the absolute values of published experimental data on the strength of mixed microstructures.MST/1901
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中炭素低合金鎬(0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V)의 變態組織과 機械的性質을 硏究하였다. 즉 martensite, 50% martensite+50%下部 bainite의 混合組織 및 下部 bainite의 세 組織에 대하여 引張性質과 衝擊靭性을 調査하고, 透過電子顯微鏡으로 微細組織을 觀察하였다. 230℃ 및 350℃에서 tempering한 경우 混合組織의 引張强度와 降伏强度가 martensite 또는 下部 bainite 各 單一組織의 그 값들보다 높았다. Tempering溫度의 增加에 따라, martensite와 混合組織에서는 引張强度 및 降伏强度가 점차 減少하였으나 下部 bainite의 경우는 別 變化가 없다가 대략 400℃ 이상의 tempering 溫度로부터 減少했다. 이들 세 組織을 450℃에서 40分 동안 tempering하여 引張强度를 145㎏/㎟ 水準으로 同一하게 한 후 衝擊靭性을 試驗 比校한 바 martensite 組織의 靭性이 가장 優秀했다. 引張試驗 및 衝擊靭性에 대한 試驗結果를 電子顯微鏡으로 觀察한 微細組織과 관련시켜 檢討하였다.
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