Surface hardening by strain induced martensitic transformation

The hardness and fatigue strength achieved by strain hardening are normally noticeable lower than those attained by thermal or thermochemical heat treatments. Strain or deformation induced martensitic transformation of residual austenite can increase the strength achieved by mechanical surface hardening processes considerably. In this paper, an approach is presented where workpieces with a high content of metastable austenite are used for hardening the surface layer. The microstructure has to be sufficiently stable, in order to ensure that the material can be machined without being changed by strain induced transformation of the residual austenite. After machining, high Hertzian contact stresses are introduced by deep rolling, so that a strain induced martensitic transformation of the residual austenite takes place. At the same time deep rolling produces the surface finish of the part. By this method, a surface hardening without a heat treatment process within the production line can be realized. A conceivable use of this method could be the production of bearings or guideways.     

Geometrically necessary dislocations,hardening, and a simple gradient theory of crystal plasticity

Geometrically necessary dislocations (GNDs) and incompatible lattice deformations are directly related and characterized by a gradient of the elastic distortion. This measure of GND density can play a natural role in non-local theories of plasticity. Issues concerning hardening and boundary conditions are addressed in the setting of a simple theory.     

双相钢板应变强化和烘烤硬化特性试验研究

近年,国内高强度钢板品种开发取得了快速的进步,双相钢等先进高强度钢板日趋系列化,同时强度等级也有了大幅度的提高。但是与国外先进钢厂相比,国内先进高强度钢板的推广应用步伐相对缓慢。该文针对C-Mn钢、高强度低合金钢、590MPa双相钢和780MPa双相钢的应变强化特性和烘烤硬化特性开展了深入的试验研究。研究结果表明,与C-Mn钢和HSLA钢相比,双相钢应变强化和烘烤硬化效应十分显著,有利于提高零件的使用强度。     

A new hardening law for strain gradient plasticity

A new hardening law of the strain gradient theory is proposed in this paper, which retains the essential structure of the incremental version of conventional J₂ deformation theory and obeys thermodynamic restrictions. The key feature of the new proposal is that the term of strain gradient plasticity is represented as an internal variable to increase the tangent modulus. This feature which is in contrast to several proposed theories, allows the problem of incremental equilibrium equations to be stated without higher-order stress, higher-order strain rates or extra boundary conditions. The general idea is presented and compared with the theory given by Fleck and Hutchinson (Adv. in Appl. Mech. (1997) 295). The new hardening law is demonstrated by two experimental tests i.e. thin wire torsion and ultra-thin beam bending tests. The present theoretical results agree well with the experiment results.     

Strengthening of crystalline substances by programmed strain hardening

Conclusions Investigations of the mechanical and physical properties of metals at different loading rates and temperatures confirmed the existence of two types of hardening which differ in their physical nature — dislocation-substructural and diffusion.Diffusion hardening is practicable under conditions of programmed loading in which the rate of increase of external stress equals the rate of diffusion transfer of lattice defects. The structure of metals hardened by programmed loading is more uniform and mechanically more stable with respect to the original as the result of diffusion redistribution of point and line defects, pinning of dislocations, and the removal of local stress concentrations because of microplasticity, leading to release of dislocation pile-ups and their movement to the boundaries. The elimination of weak places with stress concentrations and the removal of structural heterogeneities (regions of potential nucleation of brittle cracks) after programmed loading is indicated by the uniform distribution of slip traces, improvement of the plastic properties, reduction of the static cold brittleness threshold, suppression of the Bauschinger effect, etc. The existence of the effect of programmed hardening in pure metals, the increase of the resistance to deformation without reduction of the plasticity, an improvement of the plastic properties for some metals, little change in the internal friction of iron in the temperature region of the carbon peak, and other results of the investigation indicate the fundamental difference between the mechanism of programmed hardening and ordinary age-hardening.The distinguishing characteristics of the method of programmed hardening are the increased resistance to deformation and simultaneous improvement of the combination of physical properties, the higher margin of plasticity, the stability of the microstructure with respect to recrystallization, the attainment of hardening with very small remanent strain (0.01–0.5%).Physicotechnical Institute of the Academmy of Sciences of the UkrSSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No.5, pp.2–8, May, 1967.     

Extended strain hardening by a sequential operation of twinning induced plasticity and transformation induced plasticity in a low Ni duplex stainless steel

Extended strain hardening was realized by a sequential operation of twinning induced plasticity (TWIP) followed by transformation induced plasticity (TRIP) in a Fe-20Cr-3Mn-2Cu-1Ni-1Si-0.2N duplex stainless steel (DSS). As a result, the present DSS exhibited an excellent combination of strength — ductility of 900 MPa which was 75% superior to that of conventional DSSs. The deformed microstructures of the present DSS revealed that strain induced martensite (SIM) causing TRIP primarily nucleated at intersections of mechanical twins without formation of ε martensite which is an intermediate phase during SIM transformation. In addition, the sequential operation of TWIP-TRIP enables strain hardening to be extended to higher strains compared to the operation of TWIP alone.     

Hardening the surface of austenitic steels by strain hardening

Metal Science and Heat Treatment -     

Micromechanics-based strain hardening model in consideration of dislocation-precipitate interactions

A micromechanics-based model to predict yield strength and plastic work hardening is proposed. To simplify the problem, additional strengthening by dislocation-dislocation interaction is assumed to be related only to the resistance to the motion of dislocations by uniformly distributed precipitates. The interaction between the mobile dislocations and precipitate particles is facilitated in a physically based approach. The main parameters of the proposed model are the size and strength of the precipitate under different aging conditions and microstructural parameters along with the stress state around the idealized precipitate. For verification purposes, the proposed hardening model was calibrated with previously published data and applied to the prediction of the yield stress and flow curve for precipitated alloys under different aging conditions. In particular, the existence of a transient region in the hardening rate from positive to negative could be reproduced well.     

Smaller is stronger: The effect of strain hardening

Single-crystal face-centered cubic metal pillars synthesized using a focused ion beam are reported to be stronger when compressed in smaller volumes. Using in situ Laue diffraction and crystal plasticity simulations it is shown that plastic deformation is initially controlled by the boundary constraints of the microcompression tests, followed by classical crystal plasticity for uniaxial compression. Taking the stress at which the change between the two modes occurs as strength of the pillar instead of the flow stress at a fixed amount of strain, the “smaller is stronger” trend is considerably reduced, if not eliminated, and what remains is a size dependence in strain hardening. The size-dependent increase in flow stress is a result of the early activation of multiple slip systems and thus the evolution of the microstructure during compression.     

TWIP钢在不同温度变形的加工硬化行为

通过温控拉伸试验、光学显微镜、X射线衍射技术和透射电镜分析了在298、373、473、573 K温度下变形时,20Mn24Cr5Al2Ni2TWIP钢的力学性能和显微组织变化规律。结果表明,TWIP钢的强度随变形温度的升高而降低,伸长率在373 K变形时比298 K变形显著下降;在373～573 K变形时伸长率有上升趋势;温度升高,组织中形变孪晶的数量减少,孪晶交叉现象减弱。研究TWIP钢的加工硬化行为表明,TWIP钢在拉伸过程中的加工硬化指数n值随真应变的增加而增加,在低应变区温度升高n值增加。在298～373 K变形时,形变孪晶占主导作用,在473～573 K变形时,形变孪晶和动态应变时效共同作用。     

Tungsten (111) crystal strain hardening in nano-indentations

Plastic stress-strain calculations are presented for a substantial range in load versus displacement measurements made with rounded tips of Berkovich nano-indentations on tungsten (111) crystal surfaces. The computed hardness stresses are shown to be comparable to hardness measurements obtained on [111]-axis micro-whiskers; and, the subsequent indentation strain hardening is greater than, but compares favorably with, compression stress measurements made on tungsten [111]-axis micro-pillars. The indentation strain hardening is attributed to the small spacing of nucleated dislocations that proceed to form reacted 〈010〉 dislocation obstacles within the nano-scale deformation zones. The strain hardening is a larger effect than the Indentation Size Effect (ISE) measured for deeper polygonal (or conical) indentations.     

金属薄板应变硬化模型比较分析

阐述了金属薄板在塑性变形中的应变硬化行为,总结了基于增量塑性理论的弹塑性材料本构模型推导方法,重点分析了经典随动硬化模型中的Prager-Ziegler硬化模型、Mroz硬化模型、Armstrong-Frederick硬化模型、Chaboche硬化模型、Geng-Wagoner硬化模型、Yoshida-Umemori硬化模型和Teodosiu-Hu硬化模型的理论基础及其应用范围,为今后的相关研究提供参考.     

Grain size effect on strain hardening in twinning-induced plasticity steels

We investigate the influence of grain size on the strain hardening of two Fe–22Mn–0.6C (wt.%) twinning-induced plasticity steels with average grain sizes of 3 and 50 μm, respectively. The grain size has a significant influence on the strain hardening through the underlying microstructure. The dislocation substructure formed in the early deformation stages determines the density of nucleation sites for twins per unit grain boundary area which controls the developing twin substructure.