Development of an ultra-high-strength low-alloy NiSiCrCoMo steel

An ultra-high-strength low-alloy NiSiCrCoMo steel has been developed. The development work is part of a major programme at the Defence Metallurgical Research Laboratory in the field of ultra-high-strength, high-fracture-toughness steels. In this context we undertook investigations to understand the effect of solute additions on the fracture behaviour of Armco iron and Fe-C alloys. We investigated Fe-Ni, Fe-Co, Fe-Si, Fe-Mo, Fe-C-Ni and Fe-C-Co alloys for mechanical behaviour. The report by Garrison (1986) on a Fe-C-Ni-Si-Cr alloy was an important pointer to a low-alloy, ultra-high-strength steel with high fracture toughness. The material we have now arrived at is a Fe-C-Ni-Si-Cr-Co-Mo steel with tensile, impact and fracture toughness properties matching those of maraging steel 250 grade in tonnage scale melts.     

Ageing behavior of a Cu-bearing ultrahigh strength steel

On ageing at different temperatures a various combination of properties has been obtained for this Cu-bearing ultrahigh strength steel. A substantial increase in strength has been obtained at 450 °C, accompanied by a drop in percentage elongation, percentage reduction in area and toughness. At 550 °C temperature extensive -Cu precipitates have been observed. The increased strength value retained in the temperature range of 450–600 °C and a secondary hardening peak obtained at 600 °C is probably due to the formation of fine Mo carbide precipitates. The decrease in strength at 650 °C along with an increase in percentage elongation, percentage reduction in area and toughness is due to the coarsening of Cu particles and a partial recovery of matrix. At 700 °C most of the Cu precipitates become rod shaped and formation of fresh martensite with a dark contrast is observed at the lath boundaries.     

30CrNi3MoV低合金超高强钢中的马氏体相变

通过对马氏体的显微组织进行分析,并结合线膨胀试验得到的相变动力学信息研究了30CrNi3MoV低合金超高强钢中的马氏体相变特征.结果表明:淬火冷却30CrNi3MoV钢的相变产物包括低碳板条状和高碳针状两种马氏体形态,两者的形成在动力学曲线中截然分开.板条马氏体形成于Ms以下的较高温(310℃～260℃),相变过程中发生了碳的重新分配,造成富碳奥氏体微区的形成;高碳针状马氏体形成于Ms以下的较低温(260℃～170℃),由富碳奥氏体微区转变而成.板条马氏体形成速率远高于针状马氏体.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

Precracking high speed steel for fracture toughness testing

Fracture toughness testing of high speed steel, which has a high fatigue strength and low fracture toughness, is a problem because fatigue cracks are difficult, or impossible, to initiate at a maximum fatigue stress intensity of $\text{0.7 K}{}_{\mathrm{<mtext>IC</mtext>}}$, as specified. A method of initiation by the use of an electric pen and subsequent fast propagation by fatigue has been studied and a procedure developed to give accurate, reproducible values of $\text{K}{}_{\mathrm{<mtext>IC</mtext>}}$ on subsequent fracture toughness testing.     

Influences of flux-cored arc welding consumables on dynamic fracture toughness (J1d) of armour grade Q&T steel joints

Quenched and tempered (Q&T) steels are prone to hydrogen-induced cracking in the HAZ after welding. Austenitic stainless steel (ASS) welding consumables are traditionally used for welding of high hardness Q&T steels as they have higher solubility for hydrogen. The use of stainless steel consumables for a non-stainless steel base metal is not economical. In recent years, the developments of low hydrogen ferritic steel (LHF) consumables that contain no hygroscopic compounds are utilized for welding of Q&T steels The use of ASS and LHF consumables for armour grade Q&T steel will lead to formation of distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence in the dynamic fracture toughness of the armour grade Q&T steel welds. Hence, an attempt was made in this paper to study the influence of flux-cored arc welding consumables on dynamic fracture toughness ( J _1d) of armour grade Q&T steel joints. The flux-cored arc welding consumables have a significant effect on the dynamic fracture toughness of the armour grade Q&T steel welds. The joints fabricated using ASS flux-cored wires showed superior J _1d values than the joints fabricated using LHF consumables and the base metal.     

高强度热冲压钢板强韧性工艺优化研究

为改善强韧性,本文基于热冲压高强度钢板强度、塑性和韧性指标,选取加热温度、保温时间和开始淬火温度为设计因子,引入Kahn试验获得高强度热冲压硼钢撕裂强度和单位面积裂纹形核功来表征材料断裂韧性,进行多指标综合评分的L9(34)正交试验设计,以研究不同淬火工艺参数对热冲压高强钢强韧性的影响规律.结果表明:在加热温度为920~950℃、保温时间1 min、开始淬火温度为650~700℃条件下,热冲压硼钢SPFH具有优良的成形性能和强韧化指标.采用优化后工艺进行典型车身结构件热冲压试验,其撕裂强度、单位面积裂纹形核功和强韧比分别提升10.91%、20.32%和22.17%,在保证强度的基础上韧性得到了大幅度提高.     

Strain-induced formation of a gradient nanostructured surface layer on an ultrahigh strength bearing steel

In the present work, an ultrahigh strength bearing steel(AISI 52100) was subjected to surface mechanical rolling treatment(SMRT) at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured(GNS) surface layer with a mean grain size of ~24 nm at the top surface was obtained on the bearing steel, resulting in an increment of ~20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.     

Long-term service effect on the toughness of pipeline steel 17GS

A transit oil pipeline, 720 mm in diameter, 334 km in length, and made of steel 17GS, after 30 years of service was twice non-destructively tested with intelligent Pipeline Inspection Gauges (PIGs). Two segments of this pipeline were cut in order to investigate the steel structure, composition, and mechanical properties and to compare these properties with those of `archive' material. Metallographic and microprobe analyses revealed no significant difference between the `archive' and `aged' materials. Specimens for testing tensile mechanical properties, for fatigue-crack growth rate measurements, and for a Charpy test were used. All those specimens were fabricated from both the base and weld metal in longitudinal and circumferential directions. At room temperature, the ultimate strength of the aged steel was found to be 5% higher than that of the `archive' material, whereas the yield point was 18% higher for the aged material. Charpy energy changed more significantly; decreasing by approximately 40% at room temperature. In addition to conventional mechanical properties, new characteristics such as `microcleavage stress' R _MC (minimum brittle fracture stress) and the toughness index K _T were determined. It has been found that a long period of operation does not affect the values of the steel microcleavage stress R _MC and does not influence the hardening exponent significantly. The use of the local approach to fracture has shown that an increase in yield stress is the main cause of the decrease in its toughness after long-term service. It is shown that a minor (15%) increase in the yield stress results in a rise (by 40°) in the Charpy transition temperature and may cause a twofold reduction in the fracture toughness K _IC.     

Hydrogen-assisted fracture features of a high strength ferrite-pearlite steel

Up to now, the exact reason of hydrogen-induced fracture for ferrite-pearlite (FP) steel is still not fully understood. This study presents detail observations of the feature beneath the fracture surface with the aim to reveal the hydrogen-induced cracking initiation and propagation processes. Slow strain rate tensile (SSRT) testing shows that the FP steel is sensitive to hydrogen embrittlement (HE). Focused ion beam (FIB) was used to prepare samples for TEM observations after HE fracture. The corresponding fractographic morphologies of hydrogen charged specimen exhibit intergranular (IG) and quasi-cleavage (QC) fracture feature. Pearlite colony, ferrite/pearlite (F/P) boundary and the adjacent ferrite matrix are found to be responsible for the initial HE fracture and the subsequent propagation. With increasing of the stress intensity factor, fracture mode is found to change from mixed IG and QC to entire QC feature which only occurs at the ferrite matrix. No crack is observed at the ferrite/cementite (F/C) interface. This may be mainly due to the limited pearlite lamella size and relatively low interface energy.     

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.     

A model for the dynamic fracture toughness of ductile structural steel

We assume in this paper that the dynamic fracture toughness K_Id of ductile structural steels is dependent on void nucleation and void growth. The void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g were obtained by modifying the void nucleation-induced and void growth-induced static fracture toughness models, respectively, considering the effect of strain rate and local temperature. By the relationship between the void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g((K_Id)²=(K_Id·n)²+(K_Id·g)²) dynamic fracture toughness K_Id could be quantitatively evaluated. With this model the dynamic fracture toughness of two structural steels (X65 and SA440) was assessed, and the causes for the differences between the static and dynamic fracture toughness were also discussed.     

A model for the static fracture toughness of ductile structural steel

Fracture of ductile structural steels generally occurs after void initiation, void growth and void coalescence. In order for ductile fracture of structural steels to occur, energy must be spent to induce void initiation and void growth. Therefore, fracture toughness for ductile fracture should be contributed from void initiation and void growth. On the basis of this suggestion static fracture toughness (K_IC) of ductile structural steels is decomposed into two parts: void nucleation-induced fracture toughness (denoted as K_IC.n) and void growth-induced fracture toughness (K_IC.g). K_IC.n, defined as the stress intensity factor at which voids ahead of a crack begins to form, is calculated from crack tip strain distribution and void nucleation strain distribution. In contrast, K_IC.g is determined by the void growth from the beginning of void nucleation to void coalescence. Therefore, K_IC.g relates to the void sizes and void distribution. In this paper, the expression for K_IC.g is given from the void sizes directly from fracture surfaces. The relationship between K_IC.n, K_IC.g and K_IC is expressed in the form (K_IC)²=(K_IC.n)²+(K_IC.g)². The newly developed model was applied to the fracture toughness evaluation of three structural steels (SN490, X65 and SA440), and the theoretical calculation agrees with the experimental results.     

Effect of alloying additions onK ISCC of ultrahigh strength NiSiCr steel

The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theK _ISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theK _ISCC whereas molybdenum addition decreased theK _ISCC. Cerium addition did not affect theK _ISCC while the combination of cobalt and molybdenum resulted in an increase in theK _ISCC although not as much as in the case of cobalt addition. The effect of alloying elements onK _ISCC could be attributed to their effect on the critical fracture stress and yield strength.     

应用炉卷轧机开发700MPa级高强韧性钢板

700MPa级高强韧性钢板具有强度高、低温韧性好以及焊接性能优良的特点.然而,传统的铁素体/珠光体钢的屈服强度一般低于500MPa,不能满足强度要求.本研究采用现代炉卷轧机并结合控轧控冷工艺在安阳钢铁公司研制开发出700MPa级高强韧性钢板,其屈服强度大于560MPa、抗拉强度大于670MPa、延伸率大于16%、-40℃冲击功大于47J.     

遗传算法在无钴高强韧钢设计中的应用

提出一种利用残缺实验数据建立无钴高强韧钢力学性能与钢的合金成分及热处理条件关系模型的方法,依据该方法建立了人工神经网络模型,并应用跗算法,对无钴高强韧钢进行了优化设计,实践表明所提出的方法是有效的,为研制新材料开辟了一条新途径。     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

The lower bound toughness procedure applied to the Euro fracture toughness dataset

The validity of a statistical method for estimating an engineering lower bound fracture toughness in the ductile-to-brittle transition region is investigated using the Euro fracture toughness dataset generated in the European SM&T Project “Fracture Toughness of Steel in the Ductile-to-Brittle Transition Regime”. The lower bound method is based on the empirical evidence that, in the low probability regime, the cumulative failure probability function tends to be a straight line rather than a curve as is the case for Weibull distributions. The investigation demonstrates that the engineering lower bound toughness values as predicted by the method are related to a cumulative cleavage failure probability lower than 2.5%. Such bound predictions can be achieved on the basis of a small number of cleavage toughness values measured at the temperature of interest. The results confirm the validity of the method.