Effects of Si on the stability of retained austenite and temper embrittlement of ultrahigh strength steels

Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.

ON THE STRENGTHENING MECHANISM OF HIGH Co-Ni ULTRAHIGH STRENGTH STEEL

In this paper, the microstructure and strengthening mechanism of 23NiCo steel were studied by transmission electron microscopy and atom-probe field-ion microscopy. The results show that the peak strength obtained when the steel is tempered at 440455℃is due to the precipitation of coherent zones of fine carbides.The peak in toughness attained at the absence of cementite and the formation of reverted and stable austenite.Tempering at higher temperatures resulted in loss of both strength and toughness. The drop of the strength of the steel tempered at high temperature is due to precipitation coarsening and loss of coherence with the matrix.

SOLUTION TREATMENT EFFECTS IN COBALT-NICKEL SECONDARY HARDENING STEEL

Effect of the austenitizing temperature on the microstructure,strength, and toughness of 16NiCo and 23NiCo alloys was studied. With increasing austenitizing temperature, the reductions in strength and hardness are probably caused by the coarsening of M_2C carbides. The use of higher austenitizing temperature resulted in improved impact toughness due to the dissolution of undissolved (Cr, Fe, Mo)_(23)C_6 carbides.As the austenitizing temperature is increased to 1050℃ for 16NiCo and 23NiCo alloys impact toughness levels are reduced on average by 31 and 53 J/cm￣2 respectively.The use of 1050℃ austenitizing temperatures resulted in intergranular fracture for 23NiCo alloy in 482℃ aged condition. It was suggested that this grain boundary embrittlement was the result of the formation of cementite particles during ageing.

Effect of low-temperature tempering on the mechanical properties of cold-rolled martensitic steel

Cold-rolled martensitic steel is an important type of advanced high-strength steel for automobile production.With martensite as its primary microstructure constituent, martensitic steel possesses exceptional high strength despite its low alloy content.As the strength of cold-rolled martensitic steel increases, the martensite and carbon content also increases, leading to a decrease in bending properties and toughness.In this paper, the effect of various tempering parameters on the bending property and impact toughness of a quenched cold-rolled martensitic steel sheet was studied.It is found that after quenching, the ductility and impact toughness of the experimental steel are improved using low-temperature heat treatment.The optimal tempering conditions for ductility and toughness are analyzed.

2.47 GPa grade ultra-strong 15Co-12Ni secondary hardening steel with superior ductility and fracture toughness

This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain(PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness(KIC) was 29.3 MPa m1/2, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode(cup-and-cone) and the formation of about ~1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed.

Phase Transformation of a Cold Work Tool Steel during Tempering

The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_（23）C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston（ G-P） zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_（23）C_6 carbides.When ageing at 700 ℃ for more than 20 h,recrystallization occurred,which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine,which was attributed to the pinning effect of M_（23）C_6 precipitates.

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 suitable for production of long-life high-strength gun barrels with the combination of superior elevated-temperature strength and good impact toughness.

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

The hardness, tensile strength and impact toughness of one quenched andtempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both atroom temperature and at elevated temperatures were investigated in order to develop high-strengthsteel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensilestrength at elevated temperature in comparison with the commonly used G4335V high-strength gunsteel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strengthof the steel is attributed to the strong secondary hardening effect and high tempering softeningresistance caused by the tempering precipitation of fine Mo-rich M_2C carbides in the α-Fe matrix.The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, itsroom-temperature impact energy is much higher than the normal requirement of impact toughness forhigh strength gun steels. Therefore, the steel is suitable for production of long-life high-strengthgun barrels with the combination of superior elevated-temperature strength and good impacttoughness.

新一代针阀体用二次硬化合金钢的强韧性研究

使用温度更高、喷射压力更大的柴油机燃油喷射系统对针阀体的力学性能、耐蚀性和可靠性要求更高。新一代针阀体可采用强度高、热稳定性好的二次硬化合金钢制作。分别采用感应熔炼+电渣重熔和电弧熔炼+电渣重熔两种冶炼工艺试制了两种化学成分不同的二次硬化5Cr-Mo-V钢,被编为A号和B号。检测了从1020℃氮气淬火随后分别于200、400、500和600℃回火的A和B钢的硬度、冲击韧性和微观组织。结果表明:淬火和600℃回火后,A钢硬度为49.0 HRC,B钢硬度为48.5 HRC,根据洛氏硬度(HRC)换算得到钢的抗拉强度高达1600~1850 MPa,冲击吸收能量约为400 J,显微组织均为回火马氏体和碳化物;适当提高5Cr-Mo-V钢的Mo含量能抑制在二次硬化区左侧400℃回火时马氏体的分解,从而改善钢的抗回火性能。

回火温度对Mn-Cr-Ni-Mo钢调质后组织与性能的影响

以Mn-Cr-Ni-Mo低碳低合金钢为研究对象,在JMatPro软件模拟的基础上,采取组织分析与性能测试等方法研究了不同回火温度条件下的组织与性能。结果表明:试验钢经淬火后的显微组织为板条贝氏体+多边形块状铁素体+马氏体,再经600~700℃高温回火时,随着回火温度的提高,马氏体和贝氏体先后逐渐转变,碳化物析出量提升,出现晶粒细化现象,平均晶粒度提高,这些组织变化导致材料强度和硬度降低而冲击韧性上升。在640℃回火时出现二次硬化现象,导致强度有所提高,冲击韧性相对略有下降,且在0~-40℃时的冲击值无明显变化,保持较高的强-韧性匹配。Mn-Cr-Ni-Mo低碳低合金钢采用910℃淬火+640℃回火的热处理工艺方案可行,能满足试验钢生产X60、X65和X70等钢级管线管的技术要求。这种用单一钢种生产多个不同钢级的钢管是保障生产管理、降低成本和提升效益的重要途径,有助于提升管线管产品的市场竞争力。

二次回火温度对GX23CrMoV12-1耐热钢组织与力学性能的影响

GX23CrMoV12-1耐热钢为燃气轮机核心铸件用钢,热处理工艺为1050℃×4 h油冷淬火+740℃×6 h炉冷回火。对该材料一次回火后再进行二次回火处理,探究二次回火温度对该材料组织和力学性能的影响。结果表明,二次回火后材料的组织与一次回火后的组织相同,均为细小的板条马氏体。二次回火后材料的强度、硬度和延伸率均降低,且随着二次回火温度从710℃升高至740℃,材料的抗拉强度和屈服强度呈缓慢下降趋势,但延伸率呈增大趋势。在720℃温度条件下二次回火后耐热钢组织中出现了较多的粗大析出相,材料的塑性和韧性最低。二次回火后材料的冲击韧性较一次回火态有显著提升,在740℃温度条件下二次回火后材料的吸收能高达71.8 J,较一次回火态提高56%。

挤压成形和回火温度对DT300钢力学性能的影响分析

本文研究了挤压成形和不同回火温度对DT300钢力学性能的影响,并分析了各工艺条件下的影响规律,以获得最佳材料力学性能,为指导企业生产、促进DT300钢的推广应用提供了有效的试验数据。

回火温度对Cr-Mo-V系高强度钢力学性能的影响

回火温度对Cr-Mo-V系高强度钢力学性能的影响并非单调变化.在低于500℃时,随着回火温度的升高,实验钢的硬度、强度和应变硬化指数降低,塑性和韧性提高,在500-630℃的温度范围内,由于二次硬化效应,实验钢的硬度、强度和韧性变化不大.对单轴拉伸实验过程的能量分析表明,在450℃附近回火时有一能量低谷,而在585℃附近回火时有一能量高峰.在单轴拉伸状态下,断裂受裂纹扩展控制.

高CoNi合金钢中二次碳化物的析出与转化

高 ＣｏＮｉ超高强度合金钢是典型的淬火回火马氏体钢，等温回火处理产生的针状合金碳化物沉淀即二次硬化反应使材料达到高的强韧性．针状合金碳化物 Ｍ２Ｃ从马氏体基体ａ－Ｆｅ中以共格形态析出，该共格关系随过时效而失去回火温度较高时 Ｍ２Ｃ转化为稳定的合金碳化物 Ｍ２３Ｃ６和 Ｍ７Ｃ３．

回火工艺对Aermet100超高强度钢组织与韧性的影响

采用SEM、TEM等方法研究了不同回火温度对Aermet100超高强度钢的显微组织和韧性的影响。结果表明,Aermet100钢经885℃淬火后,在472～492℃温度范围内回火,显微组织为高位错密度的板条马氏体(板条M)和少量逆转变奥氏体(AR)。随着回火温度的升高,板条M宽度略有增加,韧化相AR的含量有所提高。高密度位错马氏体的存在使得Aermet100钢保持较高的韧性水平,而逆转奥氏体含量的增加则导致Aermet100钢的韧性显著提高。

二次强化高CoNi超高强度合金钢的研究近况

近年来开发的高 Co Ni超高强度合金钢 ,以其合理的合金设计、极高的力学性能和使用性能而获得广泛的关注。高 Co Ni合金钢是典型的马氏体时效钢 ,它利用时效处理析出的细小弥散M2 C合金碳化物达到二次强化的目的 ,从而获得强度与韧性的最佳配合。作者从材料开发、性能。

热处理工艺对300M超高强度钢组织和性能的影响

采用SEM、TEM等方法研究了不同回火温度对300M超高强度钢的显微组织和力学性能的影响。结果表明,300M钢经870℃淬火后,在290～320℃范围内回火,显微组织为板条马氏体、下贝氏体和残留奥氏体组成。随着回火温度的升高,板条马氏体宽度由260 nm增加到437 nm,位错密度减小,下贝氏体含量增多;合金的抗拉强度有所下降,韧性呈上升趋势,而屈服强度、伸长率和断面收缩率变化较小。当回火温度为300℃时,强度、塑性和韧性达到一个最佳匹配,合金具有最优的综合力学性能。

回火处理对铸造高速钢轧辊耐磨性的影响

通过销盘磨损试验,研究了在两体磨损条件下,两种高速钢轧辊材料在不同回火条件下的抗磨损性能。结果表明,淬火温度较低时,高速钢无回火二次硬化现象;而淬火温度较高时,淬火组织中残留奥氏体增多,回火冷却时转变为马氏体,使钢获得硬化。普通高速钢经1050℃淬火+530～560℃回火后,耐磨性好,用钛和稀土镁合金处理的高速钢经1040℃淬火+520～540℃回火后,耐磨性好。相同磨损条件下,钛和稀土镁合金处理高速钢的磨损量一般低于普通高速钢。

Cr-Mo-V系高强度钢的应力腐蚀开裂行为

用改进的WOL型试样研究了Cr-Mo-V系高强度钢在3．5％NaCl水溶液中的应力腐蚀断裂行为，并与常用的42CrMo钢进行了对比。结果表明，回火温度对实验钢的应力腐蚀开裂行为有显著影响。当回火温度较低时，Cr-Mo-V钢的KISCC较低，且随回火温度的升高缓慢提高，断裂方式主要为沿晶断裂；当回火温度超过约873K后，KISCC显著提高，断口中穿晶断裂所占的比例明显增加，断裂方式逐渐转变为穿晶型断裂为主。在相同的强度水平下，Cr-Mo-V钢的KISCC高于42CrMo钢，且在低强度水平下二者差别较大。对42CrMo钢，KISCC随屈服强度升高呈指数下降。但对Cr-Mo—v钢，则并非如此，即在强度保持不变（Rp0.2=1410—1440MPa）的条件下，KISCC随回火温度升高而明显提高。Cr-Mo—V钢和42CrMo钢的KISCC均随回火温度的升高呈指数关系增加。

3Cr-3Mo二次硬化钢的回火组织和力学性能

研究了 3 Cr- 3 Mo二次硬化钢淬火回火后的组织和力学性能。结果表明 :随回火温度的升高 ,试验钢中先后析出 M3C、M2 C和 M7C3等碳化物 ,在 5 75℃回火时硬度达到峰值 ;40 0～ 5 75℃回火后试验钢的抗拉强度约为 160 0 MPa,冲击韧性为 3 0 J/ cm2 ;回火温度高于 60 0℃时 ,强度和硬度迅速下降 ,冲击韧性增加 ;64 0℃回火时 ,以 M2 C型碳化物为主 ,抗拉强度为 110 0 MPa,冲击韧性增加至 5 5 J/ cm2 。