Experimental investigation on heat treatment of a high‐speed steel for hot rolling roll mill

The authors describe the researching results to optimise the hardening and tempering of the high carbon high‐speed steel for rolls containing 2.38%C, 5.07%V, 6.34%Mo, 5.09%Cr, 1.20%Ni, 1.17%Nb, 0.09%Ti and 0.05%RE by means of light optical microscope (LOM), scanning electron microscope (SEM), backscattered electron image (BSE), X‐ray diffraction (XRD), and hardness, tensile strength, impact toughness and wear testers. The results show that the microstructure of above casting high‐speed steel is given by a tempered martensitic matrix surrounded by eutectic carbides. Casting high‐speed steel has higher hardness quenching at 1280 K–1340 K, and it has higher hardness, tensile strength, impact toughness, and abrasive wear resistance tempering at 793 K–833 K. The comprehensive properties of casting high‐speed steel is the best while air‐cooling quenching about 1340 K and tempering about 813 K.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 2 – Mechanical property characterisation

Abstract

Mechanical property characterisation has been carried out on specimens of 16Cr - 5Ni - 1Mo stainless steel, subjected to various aging cycles. The heat treatment cycles involved solution treatment at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, tensile, hardness, impact, and creep tests were conducted. Specimens aged at 475 ° C exhibited maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, while specimens aged at 625 ° C had maximum values of impact toughness and ductility. The results were correlated with the microstructural data presented in Part 1 of this study. Softening of the martensitic matrix at 625 ° C occurs as a result of the elimination of internal stresses, the decrease in the dislocation density, and the high volume fraction of austenite which lead to the drop in values of tensile strength and hardness. The results of the study reveal that aging at 550 ° C for 4 h gives the optimum combination of strength, hardness, ductility and toughness for this steel.     

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.     

Effects of yttrium on microstructure and properties of reduced activation ferritic-martensitic steel

This study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071?wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071?wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034?wt-%.     

超高强度结构钢的组织细化及韧性改善

通过合理设计化学成分以及有效地优化热处理工艺,新研制的系列贝氏体结构钢在保证超高强度(σb>1400MPa)条件下,塑性与韧性配合良好,且V型缺口冲击能(AKV>140J)与同强度级别的马氏体钢相比提高一倍多.结合显微分析结果,深入地讨论了韧性改善的物理机制.     

Effect of Aging on Microstructure and Mechanical Property of 1900 MPa Grade Maraging Stainless Steel

The 18%Ni alloy steels provide high strength and toughness, while age-hardenable or PH stainless steels also have good corrosion resistance. This paper focuses on an investigation of the heat treatment, mechanical properties and microstructural development of a new maraging stainless steel. It is reported that the heat treatment process should consist of solution treatment and cryogenic cooling to attain a fully martensitic structure, followed by aging at 813 K. This heat treatment resulted in an ultimate tensile strength of over 1900 MPa combined with good impact toughness. Transmission electron microscopy is used to show that, for the peak-aged condition （813 K/4 h）, nano-sized precipitates, e.g. Ni3Mo and/or R-phase, and a high density of dislocations were uniformly dispersed in the lath martensite matrix. The calculated yield strength, based on a revised Orowan mechanism, is in good agreement with the test data. The steel studied has an ultimate tensile strength over 1900 MPa, excellent fracture toughness, and good resistance against over-aging and relatively good corrosion resistance as well.     

A Study of Microstructures and Properties of High‐Carbon Si‐Cr Cast Steel Containing Rare Earth and Titanium

The microstructure, tensile and impact behaviour of high‐carbon Si‐Cr cast steel containing rare earth (RE) and titanium have been determined after austempering. The additions of RE and titanium refined the primary austenite grain size resulting in improving toughness. The addition of silicon handicapped the formation of carbide and carbide‐free bainitic ferrite and carbon enriched retained austenite could be obtained in the austempering structures of high‐carbon Si‐Cr cast steel, which had excellent mechanical properties and abrasion resistance. Moreover, the basic tendency of the mechanical properties of high‐carbon Si‐Cr cast steel influenced by the austempering temperature was that the hardness and tensile strength reduced and the impact toughness and fracture toughness increased with increasing temperature. The comprehensive properties were the best while austempering at 330^oC.     

Study on dislocation density,microstructure, and mechanical properties of P92 steel for different heat treatment conditions

The impact of various heat treatment procedures on microstructure, dislocation density, hardness, tensile characteristics, and impact toughness of P92 steel was examined in the current experiment. The martensitic microstructure and average microhardness of 463 HV 0.2±8 HV 0.2 of the normalized steel were prevalent. A tempering procedure was carried out at 760 °C for a range of 2 hours to 6 hours. Additionally, an X-ray diffraction examination was carried out, and the results were used to determine the dislocation density. The normalized sample was characterized by a high dislocation density. The dislocation density was decreased by tempering of normalized samples. With an increase in tempering time, the effect of the treatment coarsened the grains, precipitates, and decreased the area fraction of precipitates. After tempering, MX, M₂₃C₆, and M₇C₃ types precipitates were found to have precipitated, according to energy dispersive spectroscopy and x-ray diffraction research. The ideal tempering period was determined to be 4 hours at a tempering temperature of 760 °C based on the microstructure and mechanical characteristics. Steel that was tempered at 760 °C for 4 hours had a yield strength of 472 MPa, an ultimate tensile strength of 668.02 MPa, and an elongation of 26.05 %, respectively.     

微量Ce元素对高铬高钴型马氏体耐热钢力学性能的影响

在不同条件下对X20Co高钴高铬型马氏体耐热钢进行热处理,用光学显微镜、扫描电镜、X射线衍射仪以及拉伸实验等手段进行表征,研究了微量Ce元素对其微观组织和力学性能的影响。结果表明,在X20Co钢的淬火过程中,添加质量分数为50×10^-6 Ce元素能促进M₆C型碳化物沿晶析出,阻碍晶界迁移,使奥氏体晶粒细化;在回火过程中能抑制M₂₃C₆型碳化物沿晶界聚集长大。同时,添加50×10^-6 Ce元素对X20Co高钴高铬型马氏体耐热钢的室温硬度、室温强度、高温瞬时拉伸强度没有显著的影响,但是使其室温韧性、塑性和高温塑性显著改善。     

Effects of aging temperature on the microstructure and mechanical properties of 1.8Cu-7.3Ni-15.9Cr-1.2Mo-low C, N martensitic precipitation hardening stainless steel

The effects of aging temperature on the microstructure and mechanical properties of a newly designed martensitic precipitation hardening stainless steel, which is 1.8Cu-15.9Cr-7.3Ni-1.2Mo-low C, N steel, for improving the toughness, ductility and corrosion resistance of stainless steel of 1000 MPa grade tensile strength were experimentally investigated. The specimen aged at 753 K for 14.4 ks has a typical lath martensitic structure with about 12% interlath austenite, while the specimens aged at 813 K and 853 K for 14.4 ks have the lamellar duplex microstructure of the reverted austenite and the aging hardened martensite. The formation process of reverted austenite is controlled by diffusion of Ni in martensite. The mean size of precipitates which are enriched with Cu increases with rising aging temperature, however, it is about 30 nm even after aging at 853 K for 14.4 ks. The specimens aged at 813 K and 853 K for 14.4 ks, in which the reversion of martensite to austenite is observed, have the excellent combinations of strength, ductility and toughness.     

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.     

Effect of Re-Nb on the strength and toughness of 3Cr2MoNiWV cast hot working die steel

The effects of Re-Nb on the hardness, tensile strength, Charpy impact energy, and fracture toughness of 3Cr2MoNiWV cast hot working die steel have been investigated. It is shown that the tensile strengths at room temperature and at 600°C increased by 19% and 22% respectively while Charpy impact energy and fracture toughness increased to 109% and 70% respectively while the hardness remain unchanged by Re-Nb modification. The results showed that Re-Nb modification can refine the microstructure of the steel, increase the volume fraction of lath martensite and retained austenite in the tested steel and change the morphology of non-metallic inclusions from bar-like to a fine nodular type. The results are explained based on the modification of the microstructure due to Re–Nb modification.     

Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V,which were subjected to a bi-lamellar heat treatment,which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions.The bi-lamellar microstructure was obtained as (i)a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization.For the bi-lamellar treatment with β-homogenization,cooling rates in the range 1-500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 ℃.The microstructures were characterized using various microscopical techniques.Mechanical testing with Vickers hardness indentation and tensile testing was performed.The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure,because of the presence of fine α-platelets inside the β-lamellae.Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions.The age hardened bi-lamellar microstructure shows a similar hardness as the very fine,as-built martensitic microstructure.The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition,which has high strength,but poor ductility.After the bi-lamellar heat treatment,the elongation was improved by more than 250 ％.Due to the very high strength of the as-built condition,loss of tensile strength is unavoidable,resulting in a reduction of tensile strength of～18 ％.     

退火热处理对TA15钛合金组织性能的影响

研究了不同的退火热处理制度对TA15钛合金显微组织、室温拉伸性能、高温拉伸性能、室温冲击韧性及硬度的影响。结果表明：在相变点以上温度退火,合金具有较高的室温、高温强度,但室温塑性、高温塑性、室温冲击韧性较低;在相变点以下温度退火,合金的室温、高温断裂强度在860℃退火时出现峰值,而室温塑性、高温断面收缩率和室温冲击韧性则随着退火温度的升高而提高;同单重退火相比,双重退火、三重退火对提高合金性能的作用不大。     

Effects of Re-B modification on the strength and toughness of 30CrMn2Si cast steel

The effects of multi-modification on the strength and toughness of 30CrMn2Si cast steel were investigated. The mechanical properties tested include tensile strength; Charpy impact toughness from room temperature to –60 °C; hardness and fracture toughness. Microanalyses were carried out by optical microscopy, transmission electron microscopy, scanning electron microscopy, and X-ray diffraction, and included the microstructure and submicrostructure of martensites, residual austenites, the size and distribution of non-metallic inclusions, and the original austenite grain size. It was concluded that, with the hardness unchanged, the fracture toughness of the modified steel was raised to 95 MPa m^1/2, 34% more than that of the un-modified steel, and the impact toughnesses at normal and low temperatures raised to 62 and 61.2 J cm^-2, respectively, 67 and 75% more than those of the un-modified steel. Furthermore, the fracture strength and yield strength of the steel were increased by over 200 MPa.     

Q960E超高强钢焊接接头组织和性能研究

目的 对Q960E超高强钢的焊接工艺进行研究以获得高强高韧的焊接接头。方法 选择超高强钢Q960E作为母材、FK1000ER120S–G焊丝作为填充材料进行MAG焊,采用改变焊接电流的方式来研究焊接热输入对焊接接头组织和性能的影响。结果 当焊接电流为155～230 A时,均获得了全焊透无明显缺陷的焊缝。随着焊接热输入的增大,焊接接头中各亚区宽度增大,其中焊缝区变化最为显著,在最小热输入条件下焊缝宽度为3.98 mm,在最大热输入条件下焊缝宽度增至5.53 mm。对焊接接头进行组织分析发现,焊缝组织主要为针状铁素体和板条马氏体;完全相变区组织主要为板条马氏体;未完全相变区组织主要为回火马氏体和部分重结晶形成的马氏体。硬度测试表明,在热影响区的回火区发生了软化现象,最低硬度仅为290HV;在完全相变区发生了硬化现象,硬度最大值可达500HV。在不同热输入条件下,焊接接头各亚区硬度变化趋势一致,焊接接头抗拉强度为995～1 076 MPa,拉伸试验均断裂在热影响区,断后伸长率为9.33%～10.21%,断裂时存在颈缩现象,为韧性断裂。随着热输入的增加,粗晶区马氏体板条束宽度增大,未完全相变区...     

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.     

Strength and toughness of clean nanostructured bainite

A nanostructured steel has been produced using a clean steel-making technique. The mechanical properties have been comprehensively characterised. The maximum strength of the material recorded was 2.2?GPa at yield, with an ultimate tensile strength of 2.5?GPa, accompanied by a Charpy impact energy of 5?J, achieved by heat treatment to refine the prior austenite grain size from 145 to 20?µm. This increased the strength by 40% and the Charpy V-notch energy more than doubled. In terms of resistance of the hardness to tempering, the behaviour observed was similar to previous alloys. Despite reducing the hardness and strength, tempering was observed to reduce the plane-strain fracture toughness.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Precipitation,microstructure and mechanical properties of low nickel maraging steel

Abstract

A maraging steel with a composition of Fe–12·94Ni–1·61Al–1·01Mo–0·23Nb (wt-%) was investigated. Optical, scanning electron and transmission electron microscopy and X-ray diffraction analysis were employed to study the microstructure of the steel after different aging periods at temperatures of 450–600°C. Hardness and Charpy impact toughness of the steel were measured. The study of microstructure and mechanical properties showed that nanosized precipitates were formed homogeneously during the aging process, which resulted in high hardness. As the aging time is prolonged, precipitates grow and hardness increases. Fractography of the as forged steel has shown mixed ductile and brittle fracture and has indicated that the steel has good toughness. Relationships among heat treatment, microstructure and mechanical properties are discussed. Further experiments using tensile testing and impact testing for aged steel were carried out.