Laser surface hardening of H13 steel in the melt case

Laser heating caused a melting layer to form on the H13 steel, which usually has bad thermal conductivity and diffusivity. Therefore, the modified Ashby–Eastering heat-transfer equation was used to provide the temperature field for laser surface hardening in the melt. When the laser hardened H13 steel through surface melting, the basic microstructure of the dendrites was surrounded by an extremely fine lamellar structure in the melt layer. It is clear that the contours of the melting point isotherms and the critical phase transition temperature of H13 in the quenched and as-received conditions were comparable in the temperature distribution field under different laser energy densities. When the laser moves on, the phase transition temperature of H13 is raised and it becomes higher than the A₁ temperature because the heating rate during laser processing is usually >10⁴ °C/s. The larger the grain size or the more heterogeneous the structure, the higher the temperature and the longer the duration required for transforming the steel into austenite.     

Three dimensional numerical model for laser transformation hardening of metals

Abstract

A three dimensional numerical model has been developed to calculate the transient temperature field during the laser transformation hardening of metal surfaces. The model takes into account the finite dimensions of the substrate, the temperature dependence of the physical properties, the laser scanning velocity, and the surface heat loss through convection and radiation. The model is used to predict the temperature distribution in the substrate and the phase transformation depth resulting from the laser surface treatment. Good agreement is demonstrated with experimental results for the laser hardening of a zirconium alloy.

MST/1867     

Laser transformation hardening of low alloy hypoeutectoid steel

Abstract

The principles of laser transformation hardening were investigated using a low alloy special steel having a microstructure of pearlite and proeutectoid ferrite. Temperature fields and phase transformations were modelled. Particular attention was paid to increases of the Ac₁ and Ac₃ transformation temperatures owing to the rapid thermal cycles produced by laser heating. Dissolution of proeutectoid ferrite is shown to control the formation of a homogeneous hardened case. Experimental data are in good agreement with the predictions of the model. A diagram was constructed which describes the case geometry and microstructure in terms of the process variables and is an aid to optimising practical processing parameters. The models are flexible and may be used for laser transformation hardening of other ferrous alloys having inhomogeneous microstructures.

MST/1606     

45 钢表面激光相变硬化改性组织及耐蚀性能

目的为了改善45钢表面状态,提高其表面性能,采用CO2激光器对其表面进行激光相变硬化处理。方法利用带有能谱的扫描电子显微镜(SEM/EDS)、盐雾试验机等,对激光相变硬化层组织及耐蚀性能进行了观察和分析。结果激光相变硬化层由熔化区、相变硬化区和热影响区三部分组成,其组织依次为:混合马氏体+未溶碳化物、针状马氏体、残余奥氏体。随扫描速度增加,耐蚀性先变好而后变差。结论激光相变硬化处理可改善45钢的表面性能,显著提高其耐蚀性能。     

圆柱体激光相变硬化三维温度场数值计算

建立了圆柱体工件激光相变硬化过程的三维传热学模型，根据此模型，采用有限差分法对圆柱体工件在激光相变硬化过程中的三维非稳态温度场进行了数值模拟，预测了42CrMo钢圆柱体工件激光相变硬化区的宽度和深度，对42CrMo钢圆柱体工件进行了激光相变硬化实验，并实测了工件激光相变硬化区的宽度和深度，研究结果表明：采用数值计算的方法，可以准确地计算出圆柱体激光相变硬化过程的温度分布及微观组织分布；预测的激光相变硬化区宽度和深度与实验测量结果符合的较好。     

Effect of laser heating on substructure of 0· 4%C steel

Abstract

It is expected that surface treatment by CO₂ laser will be useful for industrial applications in which improved wear resistance is required on selected areas of already accurately machined parts. Precision in the hardening of such surfaces using laser processes is an additional advantage. Moreover, it is possible to obtain these desirable properties for low cost construction steels. The microstructural changes within the laser heated zones of commercial grade medium carbon C45 steel were determined on thin foils from the treated regions using transmission electron microscopy. Components of the substructure are described and the influence of rapid laser heating on austenite formation and its transformation into martensite are discussed.

MST/563     

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     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Reinforcements affect mechanical properties and wear behaviors of WC clad layer by gas tungsten arc welding

This work deals with the surface analysis, mechanical properties and wear performances of the clad layer, which is made from tungsten carbide (WC) powders on SKD61 die steel by the gas tungsten arc welding method. According to the experimental results, due to the high hardness and elastic modulus reinforcements (Fe₃W₃C and M₇C₃) existing in the WC clad layer, the WC clad specimen has excellent wear performance at different sliding speeds.According to the wear analysis, wear behaviors of the WC clad layer are two-body abrasion and oxidation wear. In addition, oxidation wear dominates the wear behaviors of the SKD61 die steel specimen at different sliding speeds.     

Comparative study on microstructures and properties of Mn-containing and Mn-free bainitic steels

ABSTRACT

The effects of Mn on the microstructure and impact-abrasion wear resistance of bainitic steel were studied. Results showed that the Mn-containing steel possessed finer microstructure and higher volume fraction of retained austenite, in comparison with the Mn-free steel. This was caused by lower transformation temperature and higher strength of undercooled austenite. The weight loss of Mn-free steel varying with the impact load was larger than that of Mn-containing steel. High strength, hardness and toughness of Mn-containing steel were conducive to improving wear resistance. More retained austenite in Mn-containing steel played an active role in work hardening and hindering crack propagation. However, the portion of retained austenite that induced martensitic transformation was the same with increasing impact-wear load.     

Residual stresses in two laser surface melted stainless steels

Abstract

Residual stresses were studied in two laser surface melted stainless steels: one martensitic, Fe–12Cr–0·2C, and the other austenitic, Fe–17Cr–11Ni–2·5Mo (compositions in wt-%). Stresses were measured by X-ray diffractometry over a range of depths, processing conditions, and stress relieving heat treatments. The volume increase associated with the martensitic transformation develops compressive stresses in single tracks of the martensitic steel and modifies the subsurface stresses of the laser surface melted steel. However, interactions between tracks offset the compressive surface stresses at all but the slightest overlaps. Residual stresses in the martensitic steel are minimized by increasing the advance between tracks and are reduced to a lesser extent by increasing the beam diameter and decreasing the traverse velocity. The austenitic steel, undergoing no solid state phase transformation on cooling, develops tensile residual stresses of the order of the yield stress for all the processing conditions evaluated. Suitable stress relieving heat treatments were identified for each steel.

MST/422     

Plasma Post Oxidation of Plasma Nitrocarburized SKD 61 Steel

Plasma nitrocarburizing and plasma oxidizing treatments were performed to improve the wear and corrosion resistance of SKD 61 steel.Plasma nitrocarburizing was conducted for 12 h at 540℃in the nitrogen, hydrogen and methane atmosphere to produce theε-Fe-（2-3）（N,C） phase.The compound layer produced by plasma nitrocarburising was predominantly composed ofε-phase,with a small proportion ofγ′-Fe-4 （N,C） phase. The thickness of the compound layer and the diffusion layer are about 10μm and about 200μm,respectively. Plasma post oxidation was performed on the nitrocarburized samples with various oxygen/hydrogen ratio at constant temperature of 500℃for 1 h.The very thin magnetite （Fe-3O-4） layer of 1-2μm in thickness on top of the compound layer was obtained.Anodic polarization test revealed that plasma nitrocarburizing process contributed a significant improvement of corrosion resistance of SKD 61 steel.However,the corrosion characteristics of the nitrocarburized compound layer was deteriorated by oxidation treatment.     

Effect of Strain Rate,Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10^?4 to 10^?1 s^?1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve‐crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.     

Deformation behavior of duplex austenite and ε-martensite high-Mn steel

Abstract

Deformation and work hardening behavior of Fe–17Mn–0.02C steel containing ε-martensite within the austenite matrix have been investigated by means of in situ microstructural observations and x-ray diffraction analysis. During deformation, the steel shows the deformation-induced transformation of austenite → ε-martensite → α′-martensite as well as the direct transformation of austenite → α′-martensite. Based on the calculation of changes in the fraction of each constituent phase, we found that the phase transformation of austenite → ε-martensite is more effective in work hardening than that of ε-martensite → α′-martensite. Moreover, reverse transformation of ε-martensite → austenite has also been observed during deformation. It originates from the formation of stacking faults within the deformed ε-martensite, resulting in the formation of 6H-long periodic ordered structure.     

Effect of high temperature aging on ferrite transformation kinetics and tensile properties of type 316L stainless steel weld metal

Abstract

Tensile samples of AISI type 316L weld metal, deposited by the gas tungsten arc welding process, were heat treated at 873,973, and 1073 K, before being subjected to tensile loading at an initial strain rate of 5·5 × 10^?5 S^?1 at a temperature of 427 K. The transformation kinetics of δ ferrite was studied in detail. The activation energy for the transformation of δ ferrite was found to be 272 kJ mol^?1. Theferrite transformation kinetics datafor the above three temperatures were thenfitted into a master plot using the Dorn parameter, and its validity in this temperature range was verified by superimposing the ferrite transformation kinetics data at 948 and 1023 K on to the master plot. From examinations by optical microscopy, the weld metal was found to undergo competing transformation reactions during high temperature aging, namely, dissolution of δ ferrite, precipitation of carbidesj carbonitrides and σ phase, changes in σ phase morphology, and spheroidisation of σ phase. The effect of the ferrite transformation kinetics and σ phase precipitation kinetics on the tensile properties of the weld metal were also studied in detail. The yield stresses of all the aged weld metals were lower than that of the as deposited weld metal. The σ phase had no direct influence on changes in the yield stress. The ultimate tensile strength was affected only by the quantity and morphology of σ phase. All the competing processes (except changes in σ phase morphology) significantly influenced ductility. The increase in work hardening exponent n was dependent only on the amount of σ phase and not on its morphology.

MST/1983     

Visual analysis of solidification and δ–γ transformations in steels

Abstract

The theory of solidification of steels and the kinetics of austenite to α-ferrite phase transformation were extensively studied; however, comparatively, little information is available concerning the kinetics of the δ-ferrite to austenite transformation due to the difficulty of making in situ observations. In the present study, a laser scanning confocal microscopy with an infrared image furnace was implemented with which the in situ observations at the high temperature of the dynamic behaviour of the δ/γ grain nucleation and growth and interphase boundaries of the steels are made possible. The solidification mode of the carbon steel and the austenitic stainless steel during welding can be directly observed, and the definitive sequence of phase transformation that led to the final microstructure was detected in real time. Finally, new experimental results will be presented and compared with previous studies.     

Surface electron beam melting and alloying of tool steels

Abstract

Surface melting and alloying of D3 steel using an electron beam has been carried out to improve its surface microstructure and properties. The solution of primary carbides, together with rapid solidification and subsequent cooling, enhance the solubility of alloying elements in the γ Fe phase and thus influence the behaviour of the steel on subsequent tempering. The surface melted zone consists of dendrites without primary carbides, which is also the case for samples alloyed with WC, SiC, or Al₂O₃. When alloyed with TiC or TiB₂, the materials contain TiC or TiB₂ primary phase respectively in addition to the iron rich dendrites. Some unmelted TiB₂ particles are also present. On tempering, both electron beam melting and alloying change the secondary hardening characteristics, increasing the peak hardness and the peak hardness temperature.

MST/1194     

Applicability of Kocks–Mecking approach for tensile work hardening in P9 steel

Abstract

Work hardening behaviour of P9 steel in the temperature range 300–873 K has been examined in the framework of Kocks–Mecking (K–M) approach. At all temperatures, P9 steel exhibited two-stage work hardening behaviour characterised by a rapid decrease in instantaneous work hardening rate (i.e. θ?=?dσ/d?, where σ is the true stress and ? is the true plastic strain) with stress at low stresses (transient stage) followed by a gradual decrease at high stresses (stage III). Stage III work hardening of P9 steel was adequately described by K–M approach. The variations of work hardening parameters associated with K–M approach for stage III with temperature indicated three distinct temperature regimes. At all temperatures, good correlations between the respective work hardening parameters evaluated using K–M approach and from Voce equation and its derivative have been obtained for P9 steel.     

Heat treatment deformation of steel products

Abstract

The heat treatment deformation of steel products is reviewed and the thermal stress, which is one of the initiators of the deformation, is theoretically analysed taking into account transformation. Other initiators are also discussed, i.e. stress relaxation on heating, creep at high temperature, and change of microstructure on tempering. It was found that the thermal stress during quenching changes the profile of steel products, decreased long axis, increased short axis, and convex cross–section for non–hardenable and for marquenched or oil–quenched small steel parts, and increased long axis, decreased short axis, and concave cross–section for water/oil–quenched hardenable steels. These differences are the results of the different strain hardening rates of the various phases and the temperature distribution through the cross–section over the temperature range of martensite transformation. Some solutions to the problems of deformation are also discussed.

MST/21     

H13热作模具钢激光表面改性处理技术

介绍了H13热作模具钢的激光表面改性处理技术,分析了激光相变硬化、激光表面熔凝、激光合金化、激光冲击硬化等表面处理的特点及应用,研究了表面激光处理工艺的影响因素,以及激光在模具表面处理中的应用.讨论了表面激光改性处理存在的问题,提出了该领域的研究方向.