两次淬火对HSLA钢组织和冲击韧性的影响

研究了两次淬火+回火和传统的一次淬火+回火热处理对HSAL钢的显微组织和力学性能的影响。结果表明,在不显著降低强度的条件下,两次淬火使实验钢的冲击功明显提高,还改善了低温韧性和稳定性。两次淬火回火热处理可细化钢的组织,使原始奥氏体晶粒的尺寸和有效晶粒尺寸减小、大角度界面的密度和解离裂纹的扩展偏折频率提高。组织的细化和大角度晶界的增多抑制了裂纹的扩展,使韧性大幅度提高。     

Improvement of mechanical properties in a dual-phase ferrite–martensite AISI4140 steel under tough-strong ferrite formation

This paper has been concerned to investigate in details the mechanical properties of AISI4140 heat treatable steel under ferrite–martensite dual-phase (DP) microstructures in conjunction with that of conventional quench-tempered (CQT) full martensitic condition. For this purpose, a wide variety of ferrite–martensite DP samples containing different volume fractions of ferrite and martensite microphases have been developed using step quenching heat treatment processes at 600 °C for 20–55 s holding times with the subsequent hot oil quenching after being austenitized at 860 °C for 60 min in the same situation as to the CQT condition. The finalized tempering heat treatment has been carried out at 600 °C for 30 min for both of direct quenched full martensitic and DP samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with mechanical tests to assess the structure–property relationships in the DP and CQT heat treated samples. The experimental results indicate that the DP microstructures consisting about 7% volume fraction of fine grain boundary ferrite in the vicinity of martensite are associated with excellent mechanical properties in comparison to that of CQT condition. These observations are rationalized in terms of higher carbon concentration of the remaining metastable austenite leading to the harder martensite formation on the subsequent hot oil quenching, and so developing much harder ferrite grains as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation in the remaining austenite adjacent to the ferrite area. The higher martensite volume fraction in the vicinity of thin continuous grain boundary ferrite network has been associated with the harder ferrite formation, causing higher work hardening behavior in the short time treated DP samples. Moreover, it has been found that in order to optimize the mechanical properties of ferrite–martensite DP samples, two independently parameters should be simultaneously controlled: one is the ferrite volume fraction and the other is ferrite morphology.     

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Abstract

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.     

回火方式对调质高强度钢组织和性能的影响

为改善高强度钢的塑性和韧性,对同一种低合金高强度钢进行两种不同回火方式的调质处理,淬火+缓慢加热回火的传统调质与淬火+感应加热回火的新调质工艺,分析该工艺对钢的组织与性能的影响.利用扫描电镜和透射电镜观察组织及析出物的变化,采用X射线衍射仪分析了钢中残余奥氏体体积分数.结果表明:两种工艺下,钢的组织均为板条宽300～500 nm左右的马氏体组织,感应加热回火调质工艺处理后,板条组织明显,析出物大多约为20 nm,比传统调质处理后的细小;两种不同热处理工艺均能提高钢的屈服强度.感应加热至500℃回火后试验钢具有16%以上的延伸率,-40℃冲击功达到32 J,优于传统调质工艺处理钢板的综合性能.感应加热回火能获得更多小尺寸析出物和更多的残余奥氏体,有利于改善钢的塑性和韧性.     

热处理对C12A耐热钢的组织与性能影响

通过对铸造C12A耐热钢进行热处理,观察其微观组织,测定其力学性能。试验结果表明：正（淬）火组织为板条马氏体＋部分针状马氏体＋少量残余奥氏体,其硬度比较高,塑性和韧性不是很好;正（淬）火＋回火组织为回火马氏体,其硬度不是很高,塑性和韧性比较好,具有良好的综合性能;退火组织为铁素体,其硬度低,塑性和韧性高;通过正火＋回火,研究回火温度对其微观组织和力学性能的影响。试验结果表明：回火温度对C12A钢的组织和性能有较大影响,其硬度随回火温度的升高呈先降后升趋势。     

Hardness and toughness investigations of deep cryogenic treated cold work die steel

In consideration of good results about the application of deep cryogenic treatment (DCT) on materials, the effect on the microstructure and properties (hardness, toughness and the content of retained austenite) of a new developed cold work die steel (Cr8Mo2SiV) was examined. The execution of the deep cryogenic treatment in different processes showed a varying effect on materials. It was shown that the hardness of the DCT specimens was higher (+0.5HRC to +2HRC) whereas the toughness was lower when compared with the conventionally treated specimens (quenching and tempering). Following the DCT process retained austenite transformed into martensite, however, not completely.     

The effect of tempering temperature on mechanical properties and microstructure of low alloy Cr and CrMo steel

Two low alloy Cr and CrMo steels with similar levels of carbon, manganese and chromium have been studied to determine the effect of tempering temperature on the mechanical properties and microstructure. The quenching and tempering of steels were carried out using a high-speed dilatometer. The steels were quenched at the average cooling rate of 30 K s-1 in the temperature range from 1123 to 573 K by flowing argon and tempered at 673, 823 and 973 K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite. It was found that after tempering at 973 K the Cr steel contained only orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempering conditions, the CrMo steel shows higher strength but lower ductility as compared to those of Cr steel. It is shown that this difference results from finer prior austenite grain, substructure within matrix and precipitate dispersion strengthening, primarily by Mo2C. Transmission electron microscopy (TEM) bright- and dark-field micrographs as well as selected area diffraction pattern analysis of orientation relationship showed that the cementite precipitated from the ferrite matrix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973 K obtained ductile fracture by the microvoid coalescence mechanism.     

回火温度对两相区退火海工钢组织和性能的影响

对690 MPa级海工钢进行“淬火+两相区退火+回火”三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明：回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。     

Enhancement of mechanical properties and failure mechanism of electron beam welded 300M ultrahigh strength steel joints

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970 °C for 1 h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.     

热处理对原位自生VCp/Fe复合材料的油润滑摩擦磨损性能影响的研究

研究了热处理对原位自生VCp/Fe复合材料在油润滑工况条件下摩擦磨损性能的影响。结果表明,在油润滑条件下,经热处理后VCp/Fe复合材料的基体组织得到调整优化,其耐磨性能远大于铸态VCp/Fe复合材料的和正火45钢的;在变载荷的情况下,淬火+低温回火热处理后的复合材料的耐磨性比其它热处理工艺处理后的要好。     

Process control maps to design an ultra-high strength-ductile steel

Steel with 2.4–2.5 GPa tensile strength and elongation to fracture of 4.8–5.7%, is produced by designing a novel heat treatment identical to quenching and tempering, in less than a few minutes. Since addition of Si to Fe–Mn steel promotes the austenite stabilisation by carbon enrichment, the elongation to fracture of 0.6C–1.6Si–1.2Mn (wt-%) steel treated by different quenching and partitioning (Q&P) routes is improved. Results demonstrated by process control maps give a good overview of the final microconstituents. By using higher partitioning temperatures, the tempering of martensite, stabilisation of austenite and improvement of the mechanical properties, could effectively be accelerated. This approach results in significant time and cost reduction which makes this heat treatment attractive for industries.     

真空分级淬火对8Cr4Mo4V钢组织和性能的影响

在真空条件下对航空轴承用8Cr4Mo4V钢进行不同温度的分级淬火并采用扫描电镜观察其微观组织、用XRD谱进行相分析并测试洛氏硬度、冲击性能和旋转弯曲疲劳性能,研究了真空分级淬火对其微观组织和力学性能的影响。结果表明,真空分级淬火后的8Cr4Mo4V钢其微观组织由下贝氏体、马氏体/残余奥氏体和碳化物组成;随着分级淬火温度的提高,淬火和回火态钢中析出碳化物的数量增加,残余奥氏体的含量降低。分级淬火温度为580℃时淬火态钢中贝氏体的含量最高(达到13.87%),残余奥氏体的含量为28.59%。回火后析出碳化物的含量和洛氏硬度均为所有分级温度中的最大值,分别为4.37%和62.38HRC。真空分级淬火能提高8Cr4Mo4V钢的综合力学性能。与未分级真空淬火相比,进行580℃×10 min真空分级淬火的8Cr4Mo4V钢的冲击韧性提高了23.3%,旋转弯曲疲劳极限提高了110 MPa。     

新型贝氏体钢的组织和冲击疲劳性能研究

通过显微组织观察和冲击疲劳实验,研究了不同热处理新型贝氏体钢的组织和冲击疲劳性能.结果表明:新型贝氏体钢正火低温回火的组织由贝氏体铁素体和奥氏体组成,淬火低温回火组织为回火马氏体和残余奥氏体,正火低温回火热处理的冲击疲劳寿命高于淬火低温回火热处理的冲击疲劳寿命.分析了多冲疲劳裂纹扩展的行为,讨论了正火低温回火提高冲击疲劳的原因.     

Si对中锰钢淬火配分组织和性能的影响

将20Mn5钢和20Mn5Si2钢进行淬火和配分(Q&P)工艺处理,用扫描电镜观测其微观组织,用X射线法测量残余奥氏体量,研究了Si对其微观组织和力学性能的影响.结果表明,试验钢中的奥氏体含量明显高于传统的TRIP钢和Q&P 工艺处理钢;在相同Q＆P工艺条件下,20Mn5Si2钢比20Mn5有较多的残余奥氏体,析出物数...     

Application of quenching–partitioning–tempering process and modification to a newly designed ultrahigh carbon steel

The applicability of quenching–partitioning–tempering (Q–P–T) process to an ultrahigh carbon steel (UHCS) has been investigated by means of optical microscopy (OM), scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and mechanical property tests. The molten steel was modified with a multi-component modifier-rare earth and a low melting point alloy (Al–Bi–Sb) before casting into iron molds. Observations showed that the carbide exists as partly isolated and fine blocky structure in as-cast microstructure, indicating good effect of modification. After the Q–P–T treatment, carbon was partitioned into austenite from martensite, creating a mixture of carbon-depleted martensite, carbon-enriched retained austenite and fine carbides. This kind of microstructure leads to a much higher impact toughness, 32 J/cm², in comparison with the value, i.e., no more than 20 J/cm², of the conventional quenching and tempering (Q–T) treatment at the same hardness level. Furthermore, wear-resisting property of the steel has also been investigated. It showed that the Q–P–T treated steel has better abrasive wear resistance, about 18% increased, compared with the Q–T treated alloy under high load conditions.     

An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel

Direct quenching (DQ) process is an appropriate method in steels heat treatment field. This method enhances production rate, reduces energy consumption and decreases environment contamination. In this study hot-rolled AISI 4140 steel billets with different diameters (75, 80, 85, 100, 105 and 115 mm) and 20 m length were quenched directly in a water tank. Also some samples with similar size and composition were provided by conventional reheating, quenching and tempering (RQ) heat treatment process. The quenched samples were tempered at the temperature of 630 °C for 2 h. Mechanical properties of heat treated samples including tensile strength, yield strength, elongation, hardness and impact toughness were measured. Also, the microstructure and harden-ability of this steel were investigated under various conditions and the results were compared to RQ heat treated products. The results showed that direct quenching and tempering processes (DQ–T) is due to enhance of mechanical properties such as tensile strength and harden-ability of AISI 4140 and it is affected by various parameters such as steel temperature before quenching, water temperature, quenching time and also billet size.     

Influence of hot rolling and heat treatment on structure and properties of HSLA steel explosively clad with austenitic stainless steel

Abstract

The present work aims at studying structure–property correlations in an explosively clad HSLA steel with austenitic stainless steel of AISI 304L grade. The clad plate was subjected to hot rolling followed by a quenching and tempering treatment to achieve better mechanical properties in the base plate. Optical microscopy studies revealed that the interface between the two steels was wavy in the as clad plate and the waviness decreased substantially due to hot rolling. Subsequent heat treatment has not shown any significant effect either. The base plate had tempered martensite/bainite structure in as clad or heat treated conditions and ferrite-pearlite-bainite structure in hot rolled condition. The grains were finer and elongated near the interface. The stainless steel exhibited equiaxed grain structure in as clad, hot rolled or heat treated plates. Tensile properties and charpy impact energy of the base plate were lowered due to hot rolling and then increased substantially due to heat treatment. The microhardness was observed to be a maximum at the bond interface for all three conditions studied. The shear bond strength was the highest in the as clad condition and decreased for the rolled as well as heat treated conditions. Scanning electron microscopy fractography on shear bond specimens revealed the presence of predominantly equiaxed dimples with few regions of rubbed fracture. Quantitative electron probe microanalysis across the bond interface indicated linear change in concentrations of nickel, chromium and manganese between the levels appropriate to the clad layer and base metal.     

The effect of heat treatment on the toughness, hardness and microstructure of low carbon white cast irons

The effect of destabilisation and subcritical heat treatment on the impact toughness, hardness, and the amount and mechanical stability of retained austenite in a low carbon white cast iron have been investigated. The experimental results show that the impact energy constantly increases when the destabilisation temperature is raised from 950°C to 1200°C. Although the hardness decreases, the heat-treated hardness is still greater than the as-cast state. After destabilisation treatment at 1130°C, tempering at 200 to 250°C for 3 hours leads to the highest impact toughness, and secondary hardening was observed when tempering over 400°C. The amount of retained austenite increased with the increase in the destabilisation temperature, and the treatment significantly improves the mechanical stability of the retained austenite compared with the as-cast state. Tempering below 400°C does not affect the amount of retained austenite and its mechanical stability. But the amount of retained austenite is dramatically reduced when tempered above 400°C. The relationship between the mechanical properties and the microstructure changes was discussed.     

低焊接裂纹敏感性钢回火组织及力学性能研究

为优高强度低焊接裂纹敏感性钢的力学性能,对其热轧态钢板进行了不同温度的回火实验.通过光学显微镜、扫描电镜和透射电镜观察了回火显微组织的演变特征,并结合相应的力学性能检测手段分析了不同回火温度下显微组织与力学性能的关系.结果表明,550℃回火后屈服强度和抗拉强度较热轧态强度分别提高了115和30 MPa,平均冲击功提高了...     

The effects of heat treatment on the mechanical properties of multicomponent white cast irons

Multicomponent white cast irons contain many kinds of strong carbide-forming elements in order to obtain a very hard microstructure characterized by the presence of different carbides that are well dispersed in a martensitic matrix. The heat treatment of these products consists of high temperature austenization followed by quenching and two temperings, as required in order to increase their overall hardness and to completely eliminate residual austenite. The influence of tempering temperatures on the mechanical properties of these products, determined using tensile, hot compression and fracture toughness tests, was studied in this research work. Their corresponding failure micromechanisms were defined by means of the analysis of fracture surfaces.