Early Stages of Aging and Tempering of Ferrous Martensites

The early stages of aging/tempering of ferrous martensites have been studied over a wide range of carbon contents by numerous experimental techniques. A kinetic framework is presented which allows a correlation of many diverse observations. Three general regimes of structural change termed relaxation, aging, and tempering can be further subdivided, thus identifying two stages of aging (A1 and A2) which precede the conventional stages of tempering (T1-T3). The first stage of aging involves clustering of carbon atoms in c-oriented octahedral sites, while the second stage is associated with a modulated tweed microstructure containing high-carbon regions which may be interpreted as either advanced carbon clusters or coherent transition carbides. Alloy compositions are proposed to better correlate future complementary experiments required to determine these unusual structures more precisely.     

Strength of Initially Virgin Martensites at — 196 °C after Aging and Tempering

The compressive strength at —196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero M_s temperatures, has been determined in the virgin condition and after one hour at temperatures from —80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to account for this effect of molybdenum. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Tempering of steel

Tempering of martensitic steels involves the segregation of carbon, the precipitation of carbides, the decomposition of retained austenite, and the recovery and recrystallization of the martensitic structure. Because these several reactions overlap and occur on such a fine scale, it is only recently that our knowledge of the resulting structures has become reasonably complete. Our present understanding of the processes involved in the tempering of iron-carbon martensites and how they are affected by alloying elements is reviewed.     

Tempering of Iron-Carbon-Nitrogen martensites

The tempering behavior of ternary FeCN martensitic specimens, with a total amount of interstitials of about 5.5 at. pct and carbon and nitrogen contents between about 1.5 and 3.9 at. pct, was investigated in the temperature range 110 to 830 K. Analysis of the corresponding changes in crystalline structure (X-ray diffraction), volume (dilatometry), and hardness and enthalpy (calorimetry) revealed that the following processes occurred: (a) martensitic transformation of retained austenite between 110 and 200 K; (b) redistribution of interstitials in martensite up to 370 K; (c) formation of nitrogen containing α′ precipitates and carbon containing ε/η) precipitates between 370 and 450 K; (d) conversion of α′ nitride into γ′ nitride and coarsening of ε/η carbide between 450 and 560 K; (e) decomposition of retained austenite above 540 K; and (f) conversion of ε/η carbide into cementite above 570 K. Significant precipitation of carbon and nitrogen together, as nitrocarbides or carbonitrides, was not observed. From a comparison with the tempering behavior of binary FeC and FeN alloys of similar interstitial content, it was concluded for the ternary FeCN alloy that the transformation of the transition nitride (α′) into the “equilibrium” nitride (γ′) was advanced and that the precipitation of the transition carbide(ε/η) and its conversion into the equilibrium carbide (cementite) and the decomposition of retained austenite were retarded. Formerly Graduate Student, Laboratory of Metallurgy, Delft University of Technology.     

Tempering of Low-Temperature Bainite

Electron microscopy, X-ray diffraction, and atom probe tomography have been used to identify the changes which occur during the tempering of a carbide-free bainitic steel transformed at 473 K (200 °C). Partitioning of solute between ferrite and thin-films of retained austenite was observed on tempering at 673 K (400 °C) for 30 minutes. After tempering at 673 K (400 °C) and 773 K (500 °C) for 30 minutes, cementite was observed in the form of nanometre scale precipitates. Proximity histograms showed that the partitioning of solutes other than silicon from the cementite was slight at 673 K (400 °C) and more obvious at 773 K (500 °C). In both cases, the nanometre scale carbides are greatly depleted in silicon.

     

Tempering of Mn and Mn-Si-V dual-phase steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V d11Al-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of resid11Al stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Tempering of Mn and Mn-Si-V Dual-Phase Steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V dual-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of residual stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated.     

大型环形回火炉

介绍了车轮生产线上环型回火炉的设计和调试运行情况 ,推出了大型环形回火炉新炉型。具有双层炉膛的结构特点和炉温均匀性好 (± 10℃ )的技术特性。     

扇形齿轮开齿调质工艺研制

介绍了扇形齿轮经开齿后调质的一种工艺方法,调质时采取新型的防变形工装制具,减小了扇形齿轮开齿后调质的变形,保证了扇形齿轮的齿面硬度均匀,基体具有良好的综合力学性能,同时又满足了精加工的要求及精度。     

The Effects of Tempering Reactions on Temper Embrittlement of Alloy Steels

The effects of tempering reactions which produce molybdenum-rich carbides on the temper embrittlement of NiCrMo, NiCrMoV, CrMo, and CrMoV steels, particularly embrittlement due to phosphorus segregation, are reviewed. Molybdenum can act as an effective scavenger for phosphorus and other embrittling impurities, but the scavenging is lost when the molybdenum is precipitated in carbides as a result of continued tempering during service at elevated temperatures. This leads to very slow embrittlement, controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus, for example. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Observations of the temper embrittlement behavior, and of the effects of service exposure, in three CrMoV steam turbine rotors are also reported and are shown to be consistent with the previous results. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

The Effect of Second Tempering on Hydrogen Embrittlement of Ultra-High-Strength Steel

The objective of the present study is to enhance the resistance of hydrogen embrittlement (HE) via second tempering at 250 °C for 30, 60 and 120 minutes. Although second tempering results in a higher saturated hydrogen content for the second tempering specimens during a slow strain rate test (SSRT), it effectively reduces HE susceptibility. As the second tempering time increases, dislocation density decreases. In contrast, the size of the cementite and Mo_yC_x precipitates increase slightly. The density of Mo_yC_x precipitates increases, whereas the density of cementite remains approximately the same as the second tempering time increases. Regarding second tempering specimens, the volume fraction’s increase in Mo_yC_x precipitates, which acts as a hydrogen trap with high binding energy, plays an important role in reducing the HE susceptibility, and the decrease in dislocation density can also improve HE resistance. In addition, the growth of the interface of the cementite and matrix disperses more hydrogen, which could enhance HE resistance. The result also reveals that the cementite and matrix interface is a type of low-binding-energy hydrogen trap without plastic deformation, whereas the strain interface with interfacial dislocations is a type of high binding energy hydrogen trap under plastic deformation.

     

Tempering of steel during laser treatment

This study concentrates on the annealing effects of a laser-treated area by a successive laser pass in a plain carbon, a chromium, and a tungsten steel. Transmission electron microscopy (TEM), optical microscopy (OM), and wear and hardness measurements revealed substantial effects on the properties and microstructure of the plain carbon and tungsten steel which have a partly martensitic cellular structure after the first laser treatment. Close to the melt zone, martensite was again formed, whereas further away, nucleation of carbides was observed. The hardness increased in the former area and decreased in the latter. An analytical model sustained that the softening is due to carbon diffusion and martensite tempering. The softer zone in the tungsten steel exhibited a decrease of wear rate relative to the not-annealed zone in contrast to the plain carbon steel. Besides some diffusion, no annealing effects were found in the chromium steel which possessed an austenitic cellular structure after the first laser treatment. Formerly Graduate Students, Department of Applied Physics, Materials Science Centre, University of Groningen     

一种新型高性能非调质钢

采用合适的合金化设计方案、优化冶炼和轧制工艺,研制了一种组织为粒状贝氏体的非调质钢。该钢晶粒细小、组织均匀、力学性能优异,并且具有较好的热稳定性,能满足超高强度抽油杆对强韧性、疲劳寿命及热加工工艺性能的要求,可以替代传统的优质合金结构钢,也可以用于其它需调质处理的性能要求高的结构件。     

煤气回火热处理炉的设计与应用

介绍了煤气回火热处理炉的参数、设计方案及使用状况,重点就燃烧系统、热风循环系统和控制系统进行分析,总结了该炉的使用效果,及其在技术上和经济上的推广应用价值。     

Tempering behaviour of a dual-phase low-alloy steel

The tempering behaviour of a dual phase steel of 0.08% C, 1.21% Mn, 1.00% Si, 0.42% Cr, and 0.41% Mo composition with two different martensite contents of 30 and 52%. (obtained by intercritical treatments at 820 and 860°C, respectively) has been studied. The ultimate tensile strength decreased and percentage elongation increased continuously with increasing tempering temperature up to 600°C for both intercritical treatments. The yield strength has, however, increased up to 300°C, beyond which it decreased for the steel with 30% martensite. In contrast it remained almost constant for 52% martensite up to 300°C, beyond which it decreased. The martensite of dual-phase steel for both the intercritical treatments has undergone microstructural changes on tempering that are akin to those of fully martensitic low carbon steels. The SEM fractographs from the as-quenched specimens indicate that the tensile specimens failed by microvoid coalescence with the martensite areas appearing facetted and featureless while those for 600°C tempered condition by the formation of equiaxed dimples.     

42MnVRE非调质钢的研制与应用

在许多领域所用的钢材由于要求进行调质处理，因此工序复杂，成本升高。为此，人们从７０年代开始在不断开发一系列的非调质钢来替代某些调质钢，以达到降低成本的目的。目前，国内的模具、汽车、机械等行业的许多材料是４５调质钢，为减少工序，降低成本，本研究课题旨在...     

低温回火炉温度控制系统

介绍了太钢不锈热轧厂低温回火炉温度控制系统的组成和原理,并对其中的自动控制系统的实现作了具体阐述。