Fracture mechanisms near threshold conditions of nickel alloyed PM steel

The paper examined fractographically four nickel alloyed powder metallurgy (PM) steels with total porosity between 3 and 9%. Fracture surfaces were inspected on smooth rectangular specimens from constant stress amplitude tests under axial loading with zero mean stress, at 30 Hz frequency.

The area containing the fatigue crack origin was observed in a region located invariably at a specimen surface. In all cases, the fracture surface was composed of three different morphological appearances (regions) associated with changed proportions of particular fracture mechanisms: macrocrack initiation region where cracks propagated preferentially through particles and there was no influence of pores on the propagation paths; in the other regions (macrocrack growth and unstable crack growth) cracks propagated mainly through the sintering necks by ductile rupture from microvoid coalescence and transparticle cleavage fracture.     

Stages of fatigue fracture in nickel alloyed powder metallurgy steel

Abstract

Fatigue fracture of smooth rectangular specimens was investigated by testing at constant amplitude and zero mean stress, at a frequency of 30 Hz. Two nickel alloyed powder metallurgy steels with different contents of pores and MnS were examined. Fatigue fracture started with crack nucleation at pore interfaces by the formation of small steps. Growth of these nuclei was accomplished by stepwise crack tip blunting, which led to the formation of macrocracks. A dominant fatigue crack origin built up at the region of the largest number of macrocracks. Fatigue crack growth stages I, II, and III were found in this region.     

Effects of grain size on the properties of a low nickel austenitic stainless steel

The effect of the grain size (varying in the range of 2.5–50 m) on the mechanical properties and on the wear and corrosion resistance of a low nickel austenitic stainless steel is reviewed. In particular, the austenite-martensite transformation followed by annealing for martensite reversion in high nitrogen stainless steel is investigated. In order to study the effect of this thermo-mechanical process on grain refinement, the effect of cold reduction, annealing temperature and annealing times were analysed. After obtaining ultrafine grains, the effect of the grain size on the hardness and the tensile properties was evaluated and showed a Petch-Hall dependency in the fully analysed range (down to a 2.5 m grain size).The fatigue behaviour of the steel is studied as a function of the grain size showing a poor influence of grain refining on the fatigue resistance. An increase of both the wear resistance and of the localized corrosion resistance with grain refining is also detected. Results are compared to those of similar measurements on a standard AISI 304 steel.     

不锈钢/高硼不锈钢层状复合材料组织变化

为了解决高硼不锈钢材料变形难度大、易开裂、塑性低等问题,采用复合浇铸-热轧变形工艺,制备了以难变形金属高硼不锈钢为中间层的层状复合板,研究了复合材料在铸造、热轧和固溶处理各阶段的组织特点及芯层中硼化物的相组成规律.结果表明:含硼质量分数2%～2.5%的复合板芯层的铸态组织主要以共晶组织形式凝固,含硼较高的芯层铸态组织除以共晶组织形式凝固外,还形成大块的含有Cr2B和Fe2B的过共晶硼化物相;热变形使共晶硼化物发生破碎、细化,但过共晶硼化物的体积形状变化不大;固溶处理使覆层中细小的二次析出物明显减少,这有利于复合板的力学性能,但芯层中硼化物的形貌、数量及尺寸变化不大.     

Dislocation structures in nickel during high temperature low cycle fatigue at large cumulative strains

The development of the dislocation arrangement during high temperature low cycle fatigue in nickel polycrystals was investigated. Special attention was paid to dislocation structures in the vicinity of grain boundaries. It was found that a defined cell structure develops during progressing cyclic deformation. The motion of grain boundaries associated with high temperature cyclic deformation leads to a dislocation structure gradient in the wake of the boundaries. These structure gradients are suspected to be prone to setting off nucleation of dynamic recrystallization. Upon strain path changes the fatigued structure was observed to become gradually superimposed by a new cell structure without prior decomposition.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

废镍基催化剂中高含量镍的测定

以王水溶解试样,用氨水作沉淀剂,用硫脲和氟化钾掩蔽铁、铝和铜,在PH5～6的乙酸-乙酸钠缓冲溶液中,加入过量的EDTA溶液络合镍,以二甲酚橙作指示剂,用锌盐回滴过量的EDTA,从而求得镍的含量。     

Effect of boron on hot ductility oflow carbon low alloyed steel

Abstract

The influence of boron on the hot ductility of C-Mn-Al-Cr steel has been investigated. At <980°C M(CB)₃ precipitated out and about half of the boron content was in solution in austenite at >900°C. It was found that solute boron atoms segregate to austenite grain boundaries and occupy the vacancies induced by deformation. This prevents the formation and propagation of microcracks at boundaries and results in improved hot ductility and a reduced dynamic recrystallisation temperature.     

Thermodynamic calculation and analysis for aluminum‐alloyed high boron high speed steel

The equilibrium solidified phase diagrams of high boron high speed steel have been calculated and the vertical section of iron‐carbon pseudo‐binary phase diagrams has been drawn with different aluminum concentration. The effect of aluminum on phase diagrams and solidification microstructure has been investigated by using optical microscope, scanning electron microscopy, X‐ray diffraction, and differential scanning calorimetry. The results show that the austenite region shrinks to a small area and the δ‐iron changes into α‐iron directly during cooling process when the aluminum content reaches 1.5 wt.%. The addition of excessive amount of aluminum favors the formation of ferrite, which leads to the hardness decreasing. Moreover, excessive amount of aluminum (Al≥1.5 wt.%) will make network M₂B borocarbides tend to break. Alloying with aluminum raises the solubility of carbon in the matrix and reduces the quenched hardness. The calculation results are agreed with the ones from experimental. The calculation of phase diagrams method has been successfully used for the computation of phase equilibrium in the multi‐component high boron high‐speed steel system. The work provides a practical method for engineers and researchers in related areas.     

Application of criterion for A-segregation in steel ingots

A recently developed criterion for A-segregation in steel ingots and castings, based on a dimensionless Rayleigh number, is applied to case studies of two steel ingots. In both cases, experimentally obtained locations of A-segregation are correlated against Rayleigh numbers predicted through casting simulations. It is found that the value of 17±8 previously reported as the critical value for the Rayleigh criterion agrees well with the results of these case studies, but that 6 is an appropriate critical value if a conservative lower bound is sought. Limitations and sources of sensitivity in the application of the criterion are highlighted.     

Gigacycle fatigue behavior of a high chromium alloyed cold work tool steel

The influence of carbides and the effect of surface residual stresses (RS), resulting from heat treatment or from the grinding/polishing process, on the fatigue behavior in the gigacycle regime of ingot metallurgy produced D2 type tool steel was examined. RS were found to be responsible for the occurrence of two failure modes: Internal cracks initiating at large primary carbides (clusters) were observed in the cycle number range of 10⁵–10⁶, while in the gigacycle regime near-surface cracks originating at primary carbides caused failure, which was related to degradation of the RS by cyclic loading. Simple models were employed estimating the RS degradation process and the local fatigue strength along the specimen cross section as a function of active RS. In absence of considerable RS predominantly near-surface crack initiation was obtained.     

Inclusion-controlled high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel

Axial loading fatigue tests were carried out to study the influence of inclusion on high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel (AISI 11). The fatigue strength of 1538 MPa with endurance life of 10⁷ cycles were obtained by stair-case method. The fatigue specimens were also subjected to a constant maximum stress of 1650 MPa to investigate the relationship among inclusion origin size (10-30 μm), fish-eye size (70-130 μm) and fatigue life (10⁵-10⁷ cycles). The fatigue life was found to be dependent on the inclusion size and the crack propagating length. A compressive residual stress of 300-450 MPa turned out to be present at the specimen surface, and finally induced the interior failure mode. Further investigation into the correlation between stress intensity factors of inclusion origin and corresponding stages of fatigue crack growth and fatigue life revealed that the high cycle fatigue behavior was controlled by crack propagation. According to the fractographic investigation, two distinct zones were observed in fish-eye, representing Paris-Law and fast fatigue crack growth stage, respectively. Threshold stress intensity for crack propagation of 3.9 MPa√m was obtained from the well correlated line on the ΔK_I-log N_? graph. The fracture toughness can also be estimated by the mean value of stress intensity factor ranges for fish-eye.     

Texture evolution in hot band and annealed hot bands of low alloyed ferritic stainless steel

Abstract

The texture evolution in hot band and annealed hot bands of low alloyed ferritic stainless steel with about 11 wt-%Cr was experimentally investigated using quantitative texture analysis. While the hot band texture was composed of components of α fibre and in particular δ fibre, its microstructure was a banded structure of mostly relaxed martensite and retained ferrite. Both the texture and microstructure of the hot band was derived from partially recrystallised austenite. During single phase hot band annealing, there was a strong sharpening in the strength of the texture components of δ fibre by strain induced boundary migration of the retained ferrite and formation of fine carbide sheets leading to the persistence of ferrite banding. In contrast, two phase hot band annealing resulted in the formation of a nearly equiaxed duplex ferrite grain structure with an aggregate of precipitated carbides within the transformed ferrite grains and complete elimination of microstructural banding of the hot band, and also led to the occurrence of a texture memory phenomenon.     

Crack initiation mechanisms for low cycle fatigue of type 316Ti stainless steel in high temperature water

Low cycle fatigue (LCF) tests were performed for a type 316Ti stainless steel (SS) in high temperature water. Fatigue crack initiation behaviors in high temperature water were investigated. It was found that there existed several kinds of Ti-bearing precipitates, consisting of isolated TiN or duplex (Al, Mg)O/TiN, Mo-rich (Ti, Mo)C and Ti(N,C) in the steel. Fatigue cracks were mainly initiated at Ti-bearing precipitates, phase boundaries of austenite/α-ferrite phases and persistent slip bands (PSBs) in austenite. It is believed that synergism between the mechanical factors and electrochemical reactions played a key role in the process of fatigue crack initiation in high temperature water. Related fatigue crack initiation mechanisms for the 316Ti SS are discussed.     

Development of new low nickel,cobalt free maraging steel

Abstract

A new low Ni, Co free maraging steel containing 12Ni–3·2Cr–5·1Mo–ITi (wt-%) has been developed. The optimum heat treatment was found to be solution treatment at 1098 K for 60 min followed by air cooling then aging at 753 K for 180 min. Detailed optical and transmission electron microscopy revealed that the solution treated steel transformed to lath martensite. The optimised steel achieved an ultimate tensile strength of 1700 MN m^?2 and a yield strength of 1660 MN m^?2. The toughness measured in terms of impact energy was found to be 38 J. Fractographic analysis carried out in the scanning electron microscope showed predominantly dimpled structures indicating ductile failure. The transformation temperatures for this new steel were established using dilatometry.

MST/1398     

Cold toughness of alloyed high-manganese steel

Philadelphia, USA. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 85–89, March–April, 1993.     

Factors affecting prior austenite grain size in low alloy steel

The effect of varying normalising and hardening temperatures on the prior austenite grain size in a low alloy Cr–Mo–Ni–V steel has been examined. An initial relative insensitivity of grain size to increasing austenitising temperature was observed followed by a sudden growth of grains at approximately 1000 °C. A detailed study of the precipitates in the steel showed the presence of a bimodal size distribution of vanadium carbides. The grain size increase is attributed to a decrease in volume fraction and an increase in size of V₄C₃ particles with increasing temperature.