Characterization of a New Fe-C-Mn-Si-Cr Bearing Alloy: Tempered Martensite Embrittlement Susceptibility

Bearing steels containing 1% C and 1.5% Cr have been the usual material of choice for machine components submitted to rolling and contact fatigue, for more than a century. As a rule these steels are quenched from the intercritical gamma + carbide region and tempered at low temperatures (less than 250 °C), in order to retain the high hardness of the martensite matrix and avoid the tempered martensite embrittlement (TME) phenomena, which compromise the toughness of steels tempered at higher temperatures. A new high Si alloy was developed for bearing applications. The inhibiting and/or retarding effect of Si on the kinetics of cementite precipitation leads to a higher temperature of TME occurrence, allowing the tempering of the components at a higher temperature, thus increasing the toughness, without sacrificing the high hardness. The purpose of this work was to confirm the TME resistance of the new alloy. In this work, impact tests result for commercial SAE/AISI 52100 (0.25% Si) and for a modified 52100 containing 1.74% Si, were compared. No evidence of TME was detected on the Si-modified steel.     

Correlations between microstructure and surface properties in a high nitrogen martensitic stainless steel

Nitrided and tempered AISI 410S stainless steel was tested under corrosion–erosion conditions and compared to conventional AISI 420 martensitic stainless steel. The corrosion–erosion resistance of the nitrided specimens was higher than that of the AISI 420 steel when tempered at 200 °C, but it decreased with tempering temperature in the range between 200 and 600 °C. The higher corrosion–erosion resistance of the high-nitrogen steel was credited to a more homogeneous distribution of chromium in martensite and a lower number of coarse second-phase particles, especially for tempering temperatures below 550 °C. The hexagonal ϵ-nitride was identified in specimens tempered at 200 °C, while finely distributed cubic CrN nitrides were observed in specimens tempered between 400 and 600 °C. Hexagonal Cr₂N nitrides were observed at 550 and 600 °C. These coarse, high-chromium precipitates were responsible for the drop in corrosion resistance of the nitrided specimens.     

1Cr21Ni5Ti双相不锈钢不同温度时效脆化倾向研究

对1Cr21Ni5Ti双相不锈钢在400~600 ℃之间进行时效处理,测试其冲击韧性和力学性能,结合断口形貌和金相组织观察,研究1Cr21Ni5Ti 双相不锈钢不同温度时效脆化规律,提出抑制缓解脆化的工艺措施。结果表明:1Cr21Ni5Ti双相不锈钢在550 ℃时效的脆化倾向较小,脆化倾向最敏感的时效温度区间为450~500 ℃,在此区间时效时,铁素体脆化而发生解理或准解理断裂;同时,高的Ti/C比值增大450~500 ℃之间的脆化倾向,但高的Ti/C提高奥氏体的稳定性,显著降低600 ℃时效脆化倾向。在上述基础上,研究证明增加锻造比使奥氏体相产生足够变形并使其均匀分布,能有效阻止铁素体脆性裂纹的扩展,可以较大程度地降低脆化倾向。     

Tempered martensite embrittlement in a 32NiCrMoV125 steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.     

Microstructural Characteristics and Mechanical Properties of Low-Alloy,Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV-and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature,regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.     

Elevated-temperature properties of one long-life high-strength gun steel

The hardness, tensile strength and impact toughness of one quenched and tempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both at room temperature and at elevated temperatures were investigated in order to develop high-strength steel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensile strength at elevated temperature in comparison with the commonly used G4335V high-strength gun steel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strength of the steel is attributed to the strong secondary hardening effect and high tempering softening resistance caused by the tempering precipitation of fine Mo-rich M2C carbides in the aaaaaaaaaaaaaaaa-Fe matrix. The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, its room-temperature impact energy is much higher than the normal requirement of impact toughness for high strength gun steels. Therefore, the steel is s     

回火温度对二次硬化马氏体不锈钢组织和性能的影响

二次硬化对回火温度非常敏感,研究了某含Mo二次硬化马氏体不锈钢在250~650 ℃回火时组织和性能的演变过程,并利用XRD、SEM、TEM以及冲击测试等手段分析了显微组织与力学性能之间的关系,着重讨论了试验钢二次硬化与残留奥氏体增韧机理。结果表明: 480~500 ℃回火时,试验钢同时出现了二次硬化和回火脆性现象,宏观硬度达到了56 HRC以上,冲击性能为14 J·cm^-2左右,显微组织主要由纳米级合金碳化物、板条马氏体以及残留奥氏体构成,其中纳米级合金碳化物弥散强化引起了二次硬化,体积分数约为10%的残留奥氏体有利于提高钢的冲击性能。而在上述温度区之外低温和高温回火时,试验钢均具有较高的冲击性能,但宏观硬度相对较低。     

回火温度对高氮不锈轴承钢显微组织与力学性能的影响

采用万能拉伸试验机、冲击试验机、光学显微镜、XRD、SEM和TEM等对高氮不锈轴承钢Cronidur 30不同回火温度下的显微组织和力学性能进行了研究和分析。结果表明:高氮不锈轴承钢Cronidur 30在150~500 ℃回火时的显微组织为回火马氏体+碳氮化物+残留奥氏体,高于550 ℃回火后基体逐渐转变为回火索氏体,同时析出相逐渐聚集、长大;随着回火温度的升高,强度和硬度总体上呈现先下降后升高再下降的过程,而冲击性能反之,在450 ℃回火时,碳化物M₂₃C₆和氮化物Cr₂N析出明显,此时产生二次硬化现象,其抗拉强度可达2133 MPa。400 ℃回火试样发现有极少量富Cr-Fe-Mo的析出相(σ相),显著降低其冲击性能,500 ℃回火时残留奥氏体分解、转变导致冲击性能略有降低。     

Understanding the parameters controlling friction stir welding of AISI 409M ferritic stainless steel

Empirical relationships were developed to predict the tensile strength and impact toughness of the friction stir welded AISI 409M ferritic stainless steel joints by incorporating process parameters, namely rotational speed, welding speed and tool shoulder diameter. The experiments were conducted based on a three factor, five level, central composite rotatable design matrix. Sensitivity analysis was carried out and compared with the relative impact of three process parameters on tensile strength and impact toughness to verify the measurement errors on the values of the uncertainty in estimated parameters. The results indicate that the welding speed has the greatest inuence on tensile strength and impact toughness, followed by rotational speed and tool shoulder diameter.     

氮对马氏体不锈钢00Cr13Ni4Mo组织和性能的影响

采用热膨胀仪、光学显微镜以及CM12型透射电子显微镜研究了添加0.04%N(质量分数)对00Cr13Ni4Mo马氏体不锈钢相变以及正火和回火后不锈钢组织变化的影响;通过拉伸、冲击实验和阳极极化曲线测定研究了N对正火和回火后马氏体不锈钢力学性能以及点蚀点位的影响。结果表明:1050℃正火后,N全部固溶于马氏体基体中,有效提高了实验钢的强度,同时降低了韧性;550℃以上回火后,在马氏体板条内部以及板条之间形成逆变奥氏体,有效提高了马氏体不锈钢的塑性和韧性;N抑制逆变奥氏体的形成,从而抑制了不锈钢在回火过程中的软化;同时,回火过程中,Cr2N在马氏体板条界面及内部大量析出,造成不锈钢韧性和点蚀点位下降。采用传统的正火+Ac1温度以上回火热处理工艺不利于含N马氏体不锈钢获得良好综合性能。     

Use of stainless steels in bus coach structures

This study focuses on weld integrity of stainless steels in bus coach applications. Safety aspects have been studied based on fracture mechanics and impact toughness testing. Fatigue resistance of welded rectangular hollow section (RHS) profiles was evaluated according to the Eurocode 3 (1992) fatigue standard. Corrosion resistance was studied by salt-spray chamber tests in a deicing salt atmosphere and by field testing for 3 years under an urban bus. The mechanical tests show that austenitic stainless steel EN 1.4310 (AISI 301) is a superior material, and a low-C 12% Cr alloyed stainless steel EN 1.4003 is also a competitive material in bus coach applications. According to the life cycle cost (LCC) calculations, stainless steels are competitive compared with carbon steels or aluminum.     

热强碱介质中耐磨高锰钢冲击韧度的研究

采用浸泡腐蚀试验和腐蚀前后的冲击试验并借助于扫描电镜观察,研究了热强碱介质中高锰钢的腐蚀速率和冲击韧度.结果表明,高锰钢的平均腐蚀速率显著高于中碳铬合金钢,腐蚀过程中高锰钢有较严重的碱脆开裂现象.随着热强碱腐蚀时间的延长,高锰钢的冲击韧度明显降低,碱脆开裂是造成高锰钢冲击韧度下降的主要原因.在伴有冲击的热强碱腐蚀磨损工况,高锰钢有过早失效的危险.     

Permeation measurements to study hydrogen uptake by steel electroplated with zinc-cobalt alloys

Hydrogen permeation measurements were performed to investigate hydrogen uptake by a steel substrate when electroplated with zinc-cobalt alloys. The influence of plating bath composition and pH were studied and the effect of absorbed hydrogen on embrittlement of high strength steel was measured in slow strain rate tests. It was shown that the majority of the hydrogen generated during electroplating was either evolved from the surface or trapped in the coating. Only a very small proportion, equivalent to 0.1-0.6% of the total charged passed, was absorbed by the steel and was responsible for causing embrittlement. The average amount of hydrogen required to cause hydrogen embrittlement failure of quenched and tempered AISI 4340 steel was 1.5 × 10⁻⁷ g H atoms cm⁻². There was a linear relationship between the rates of hydrogen uptake from the different coatings and the extent of hydrogen embrittlement produced in the high strength steel. In certain conditions, a cobalt-rich layer formed at the steel/coating interface and this layer was shown to be beneficial in restricting hydrogen uptake and embrittlement.     

35CrNi3WA钢的热处理工艺与力学性能

35CrNi3WA钢螺栓经调质处理后要求检测其强度、塑性和韧性,其热处理工艺已经成熟。但是,该螺栓的韧性指标由Aku2改为Aku5以后,按原先工艺调质处理的螺栓其冲击韧性往往达不到要求。试验表明,其原因是钢的回火脆性所致。     

热处理工艺对0Cr13Ni5Mo钢焊接接头组织和性能的影响

对马氏体不锈钢0Cr13Ni5Mo焊接接头经过1000℃油淬后,分别进行了600℃,620℃+600℃,400℃回火。通过显微组织分析、拉伸试验、冲击试验和硬度检测对3种焊接接头的组织和力学性能进行了研究。结果表明,3种焊接接头中焊缝组织粗大,硬度最高;焊接接头的韧性低于相应热处理状态下母材的韧性;随着回火温度的降低,韧性下降,强度提高。二次回火比一次回火组织更加细小,强度和韧性更好;620℃+600℃二次回火后焊接接头具有比较理想的综合力学性能。     

Hydrogen embrittlement of high strength steel electroplated with zinc-cobalt alloys

Slow strain rate tests were performed on quenched and tempered AISI 4340 steel to measure the extent of hydrogen embrittlement caused by electroplating with zinc-cobalt alloys. The effects of bath composition and pH were studied and compared with results for electrodeposited cadmium and zinc-10%nickel. It was found that zinc-1%cobalt alloy coatings caused serious hydrogen embrittlement (EI 0.63); almost as severe as that of cadmium (EI 0.78). Baking cadmium plated steel for 24 h at 200 °C gave full recovery of mechanical properties but specimens plated with zinc-1%cobalt and then baked still failed in 89% of the time of unplated controls. It was shown that hydrogen uptake and embrittlement could be controlled by depositing thin layers of cobalt or nickel at the steel/coating interface. For example, the least embrittlement was caused by zinc-10%nickel (EI 0.037) due to a nickel rich layer with very low hydrogen diffusion coefficient that formed during the initial stages of electroplating. Similarly, a 0.5 μm nickel layer was effective in lowering the embrittlement caused by zinc-1%cobalt to that of zinc-10%nickel. Furthermore, a 0.5 μm cobalt layer deposited before a zinc-1%cobalt coating gave virtually 100% recovery of mechanical properties after baking.     

回火温度对3Cr3Mo2NiW钢力学性能和断口形貌的影响

研究了3Cr3Mo2NiW钢力学性能和断口形貌随回火温度的变化。结果显示,随着回火温度的升高,试验钢的硬度降低,韧性增加,550 ℃回火时出现二次硬化现象;600 ℃以上回火,硬度明显降低,韧性大幅度增加;700 ℃回火态试样未冲断。淬火后,随着回火温度的升高,试验钢的基体组织逐渐转变为回火马氏体、回火屈氏体和回火索氏体。300~600 ℃温度区间内回火试样的断裂方式为准解理断裂,高温回火试样的断裂方式为韧性断裂,不同温度回火后得到的显微组织和碳化物对试样的冲击韧性有较大影响。     

Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.     

Effects of low-temperature aging on AISI 444 steel

The consequences of aging at 400 and 475 °C on the mechanical properties, corrosion resistance, and magnetic properties of the ferritic stainless steel (SS) AISI 444 were investigated. Age hardening was measured as a function of aging time at both temperatures and was found to be more intense at 475 °C. The localized corrosion susceptibility increased, while the impact toughness decreased with aging time. These two effects were also more important at 475 °C. Unlike duplex SSs, AISI 444 did not present any variation in coercive force or Curie temperature with aging time. The effects on the Mössbauer spectra were also determined and analyzed.     

SCC MECHANISM OF STAINLESS STEEL AISI 321 IN ACIDIC CHLORIDE SOLUTION

It is verified that stainless steel AISI 321 is in the active anodic dissolution state in 0.5mol/L HCll+0.5 mol/L NaCl solution at 55℃.The SCC of the steel in the solution cannot be reasonably explained by passive film rupture-nepasivation theory and by hydrogenembrittlement theory.There are evidences that the fracture stress at the tip of the cracksis reduced by anodic dissolution due to its role in relieving strain hardening layer at cracktip.