Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Study of welding characteristics of 0.3C–CrMoV(ESR) ultrahigh strength steel

A new ultrahigh strength low alloy steel 0.3C–CrMoV(ESR), having an ultimate tensile strength and 0.2% proof strength of above 1,700 and 1,500 MPa, respectively, in quenched and tempered condition, was developed primarily as a cost effective material for space launch vehicle applications. Welding is a major step in the fabrication of most of the pressure vessels, structures and equipments. Steels with carbon equivalent in excess of 0.40 wt% show a tendency to form martensite on welding, and therefore are considered difficult to weld. 0.3C–CrMoV(ESR) steel has a carbon equivalent value of nearly 1.0 that classifies it as a ‘very difficult to weld’ steel. In addition it has a niobium content of about 0.10% and a vanadium content of 0.25%. It is known that niobium content of more than 0.02 wt% has a deleterious effect on the toughness properties of low carbon welds. It has also been reported that the effect of niobium on weld metal toughness is more deleterious in the presence of vanadium. Hence, in the present study, the properties of the weldment of this new steel under different heat treatment conditions (HT-1 and HT-2) have been studied. In HT-1 condition, the plates were welded in hardened and tempered condition and no further heat treatment was given after welding, while in HT-2 condition, the annealed plates were subjected to welding followed by hardening and tempering heat treatments. For HT-1 condition, only tensile properties were evaluated. The welded plates under HT-2 condition were evaluated for tensile properties, fracture toughness, residual strength and microstructure features.     

Untersuchungen zur Wälzermüdung nach induktivem Kurzzeitanlassen

Ball bearing fatigue after inductive short time tempering To improve toughness, machiniability and to avoid distortion under operating conditions inductive hardened components often were additional tempered in an oven. As a general rule tempering is applied when grinding is required. According to tempering conditions the present hardness, toughness and the microstructure will change. In the present studies it is shown, that with optimized inductive short term tempering conditions it was possible to achieve even better mechanical properties in comparison with oven tempered components. Ball bearing fatigue testings showed an improvement of wear resistance. The achieved findings opens the possibility for numerous applications to exchange the previous oven tempering through the more rapid and more low‐cost inductive short‐term inductive surface layer hardening.     

Effect of tube spinning and subsequent heat treatments on strength,microstructure and residual stress state of AISI/SAE type 4140 steel

Abstract

In the present study, the effects of deformation percentage (23, 30, 50 and 66%) and subsequent stress relief and tempering heat treatments on the mechanical properties, residual stress state and microstructure of AISI/SAE type 4140 steel tubes manufactured by forward spinning were evaluated. Mechanical properties were determined by means of hardness and tensile tests. The tangential component of the surface residual stresses was determined by a slitting method. Plastic deformation of the metal during spinning refined and elongated the grains in the direction of metal flow, following a spiral path, resulting in improved mechanical properties. Tensile and yield strengths, as well as hardness, were all increased as a function of increasing percentage deformation. With stress relieving, the strength values were enhanced, whereas a slight decrease in hardness took place. Stress relieving did not change the microstructure considerably, whereas tempering resulted in a partially recrystallised microstructure, removing the effect of plastic deformation. The tangential residual stresses were tensile, and those of the as deformed tubes increased with an increasing amount of deformation up to 50%, then tended to decrease. The magnitude of the residual stresses decreased with stress relieving heat treatment, while tempering reduced the residual stresses to negligible levels.     

Effect of tempering on nitrided and deformed austenitic stainless steel

Abstract

The effect of tempering on nitrided austenitic stainless steel AISI 316 has been studied. Nitrided specimens (with 0.4 wt-%N) were tempered for short times at temperatures up to 900°C and the results show a small effect on the microstructures and mechanical properties. The strength is consistent with a Hall–Petch relationship dependent on nitrogen content in solution. The effect of tempering has also been studied on cold and hot deformed nitrided specimens. In these cases, tempering had a range of different effects on the microstructures and mechanical properties. Specimens that are tempered before cold rolling showed a continuous decrease in strength as the tempering temperature increased, while specimens cold rolled and then tempered had a maximum strength at 550°C. Specimens with 0.4 wt-%N subjected to tempering followed by hot deformation also showed a maximum strength at similar tempering temperatures. The nature of these changes has been analysed and mechanisms have been proposed that relate microstructural effects and properties.     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901     

Mathematical Model for Tempering Time Effect on Quenched Steel Based on Hollomon Parameter

1. IntroductionTempering is an absolutely necessary process in whichthe modification of the microstructure, the improvementof mechanical properties and the removal of the stressin quenched steel have been achieved. With the develop-ment of technology & equipment[1], and the advancementof automation for heat treatment industry, the temper-ing time effect, i.e. the variation of the strength & hard-ness of quenched steel with the tempering time duringtempering, is a necessarily considered factor[…     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

XCQ16-1车桥用钢调质热处理工艺的研究

为了提高整体式车桥用钢的综合力学性能,对XCQ16-1钢进行了调质热处理工艺研究.通过材料单向拉伸、冲击和硬度等试验,研究了不同回火温度、回火保温时间、淬火温度和淬火保温时间对XCQ16-1钢力学性能的影响规律,制定了试验条件下的调质热处理工艺,并分析了不同工艺参数对材料组织的影响规律.试验结果表明:回火工艺对XCQ16-1钢组织和力学性能的影响比较大,随回火温度的升高和回火保温时间的延长,材料的强度性能下降,塑性和韧性指标上升.经860℃保温30min淬火+470℃保温90min回火调质处理后,该材料可获得良好的综合性能.     

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 swaging on hydrogen charged tensile fracture of a 12 wt-%Cr steel

Abstract

The 12 wt-%Cr secondary hardening steel considered in this work is being evaluated for use in the first wall of fusion reactors. As the service temperature can approach 500°C, the microstructure of greatest interest has been a quenched and tempered structure obtained by tempering at 750°C after air cooling from the austenitizing temperature of 1050°C. This structure is susceptible to grain boundary failure whether internal hydrogen has been introduced by cathodic charging or not. In the uncharged condition failure is ductile, but follows prior austenite grain boundaries. Hydrogen charging results in a severe loss of ductility, and tensile fractures which are 30% brittle intergranular. This susceptibility to grain boundary fracture has been attributed both to phosphorus segregation to these grain boundaries and to a nearly continuous array of grain boundary carbides. This tendency for grain boundary fracture can be eliminated and the embrittlement associated with the introduction of internal hydrogen greatly reduced by swaging and subsequently retempering the quenched, and tempered microstructure. The improved properties of the swaged and retempered conditions are attributed to the effects of swaging on the prior austenite grain boundary structure and the orientation of the grain boundaries with respect to the tensile axis.

MST/376     

热处理对激光选区熔化成型高速钢组织和力学性能的影响EI北大核心CSCD

基于激光选区熔化(selective laser melting,SLM)技术,采用加热打印基板和低功率慢扫描的打印策略,制备了近全致密、低缺陷的高速钢样品;对比分析了固溶淬火及1~4次高温回火等热处理工艺对高速钢显微组织及力学性能的影响。结果表明:SLM极高的熔融/冷却速率产生了细晶奥氏体组织,解决了高速钢中常见的粗大莱氏体组织和网状碳化物问题。固溶淬火处理后高速钢组织由马氏体和残余奥氏体组成。随后在数次高温回火过程中,高速钢逐渐向回火马氏体转变,并析出大量微米级和纳米级MC型碳化物。在马氏体相变强化和MC型碳化物沉淀强化作用下,固溶淬火+3次回火的Tempered-Ⅲ样品硬度60HRC,抗弯强度3621 MPa,弯曲断裂应变为10.1%,获得硬度、强度和韧性匹配较佳的综合性能。继续增加回火次数则导致部分碳化物长大,使得高速钢弯曲断裂应变有所降低。通过SLM技术结合固溶淬火+高温回火,能够充分发挥细晶强化、相变强化和沉淀强化效果,为高强高韧复杂形状高速钢零件的快速制备提供了新途径。     

Fracture toughness of M2 and H13 alloy tool steels

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels having compositions (wt-%) lC–4Cr–5Mo–2V–6W (AISI M2 high-speed steel) and 0·35C–5Cr–1·5Mo;amp;#x2013;1V (AISI H13 hot-work steel) was investigated. In the as-hardened condition, the H13 steel has a higher fracture toughness than M2 steel, and the latter steel is harder. In the tempered condition, the H13 steel is again softer and has a higher fracture toughness than M2. There is a decrease in fracture toughness and an increase in hardness when the austenitizing temperature is above I050°C for M2 steel and above 1100°C for H13 steel, in both the as hardened and hardened and tempered conditions. The fracture toughness of both steels was enhanced by reducing the grain size and increasing the overall carbide volume in the matrix. The steel samples of average grain diameter ≥40μm exhibit 2–3 MN m ^?3/2 lower fracture toughness than samples of average grain diameter ≤15 μm. A high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. Tempering improved the fracture toughness of M2 and H13 steels. The present results are explained using observations of changes in the microstructure and the modes of fracture.

MST/468     

Beitrag zum Verfestigungsverhalten der Stähle durch Kaltverformung

Hardening Behaviour of Steels by Cold Working Contrary to common opinion work hardening increases with higher initial hardness (strength) of steel. This is proved by analysing stressstrain curves of tensile tests at overeleastic load carried out with normalized, hardened and tempered steel. Test results taken from literature are supported by own experiments. Practical application of this fact is discussed very shortly.     

硅对烧结M3∶2高速钢回火相及力学性能的影响

为研究硅对烧结高速钢力学性能的影响,采用真空热处理炉在1230℃对含0.4%碳粉及硅添加量在0～3.0%的M3∶2高速钢进行烧结,在500～600℃对烧结后的试样进行二次回火.采用X射线衍射仪,扫描电镜及能谱分析等手段研究了不同硅添加量对回火态高速钢的物相特征及力学性能的影响规律.物相分析结果表明,添加硅后,硅主要分布在回火马氏体及M6C碳化物中,而在MC碳化物中含量较低.回火马氏体组织中硅的含量随硅添加量的增加而增加,同时,硅的添加还显著影响回火马氏体组织中铁素体相的晶格常数,在硅添加量为0.7%时,铁素体相达到最大的晶格常数.力学性能测试结果表明,适量硅的添加可提高二次硬化效果,添加0.7%硅的高速钢在550℃二次回火后获得了最佳的硬度和弯曲强度.     

Hydrogen-induced embrittlement of nickel-chromium-molybdenum containing HSLA steels

ABSTRACT

Several advanced nickel-chromium-molybdenum high strength lowalloy steels newly developed by our research team exhibit excellent mechanical strength, toughness and hardenability. However, the phenomenon of hydrogen-induced embrittlement will easily occur for these high strength steels. In this research, the hydrogeninduced embrittlement of 8625-Modified steel (8625M steel) was studied. Experimental results show that the dominant hydrogen trapping site of the 8625M steel is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogens trapped in the dislocations are diffusible. The as-quenched 8625M steel has the highest dislocation density and accordingly the highesthydrogen content after hydrogen charging. This makes the asquenched 8625M steel exhibit severe hydrogen embrittlement. After tempering at 200°C and 300°C, the dislocation density drops, and hence these tempered specimens have lower ultimate tensile strength loss. After 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again, being ascribed to the formation of needlelike and film-like cementite which will weaken the strength of martensite. After 500°C tempering, the 8625M steel has the lowest dislocation density, and the inter-lath cementite become discontinuous and spheroidal, making the 500°C tempered specimen have the lowest ultimate tensile strength loss and the highest elongation after hydrogen charging in this study.     

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。     

Microstructure and mechanical properties of as-deposited and heat-treated additive manufactured 9Cr steel

Wire arc additive manufacturing technology has been applied to fabricate 9Cr ferritic/martensitic steel. The steel is widely used in the power industries because of good performances. The effects of different heat treatment conditions on microstructure, hardness, tensile properties, and Charpy impact toughness were investigated. The results show that the microstructure of the as-deposited condition consists of untempered lath martensitic with high strength and low toughness. It was found that heat treatment can change the microstructure characteristics. Moreover, samples after heat treatment have been observed with high elongation and impact toughness but relatively low hardness and tensile strength. The better combination of strength, ductility and microstructure were obtained for the normalising temperature of 1323?K and tempering temperature of 1033?K.     

The effects of heat treatment variables on the microstructure and properties of ultra-high strength steels differing in titanium content

The influences of different austenitizing and tempering temperatures on the microstructure and properties of three experimental ultra-high strength steels (UHS) have been investigated. The steels had different Ti content and were subjected to austenitizing treatment at 900, 1000, 1100 and 1200°C followed by oil quench and tempering at 200, 300, 450 and 600 °C. It has been found that the high temperature (1100 and 1200 °C) austenitizing treatments, alter both microstructure and properties, and depending on the subsequent tempering temperature, may have a beneficial or detrimental influence upon the mechanical properties. Addition of up to 0.011 wt% Ti to the steel composition improves hardness, toughness and tensile strength. This improvement in mechanical properties is obtainable with any subsequent heat treatment. For higher Ti content (0.089 wt%), although some further improvement in hardness and tensile strength was obtained, significant degradation in toughness was achieved, particularly when the steel was subjected to high temperature austenitizing and tempering treatment.