Rotating bending fatigue properties of two case hardening steels after nitriding treatment

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 10⁷ cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000 Hv and compressive stress of 400 MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.     

Strain hardening features of ferrite-martensite steels

A model is suggested for strain hardening of ferrite-martensite steels taking account of the stereology of structural components and redistribution of strains between them during loading. Analytical expressions are obtained making it possible to calculate the yield stress for a two-phase steel at any instant of deformation and the value of true equilibrium strain depending on such structural factors as the volumetric proportion of martensite, its carbon content, and the size and distribution of ferrite grains and martensitic areas. Experimental and calculated data are compared.Translated from Problemy Prochnosti, No. 6, pp. 43–48, June, 1990.     

Carburization of high-temperature steels: A simulation-based ranking of carburization resistance

Carburization is a failure mechanism affecting equipment, such as furnace tubes, operating at high temperatures. Carburization simulations were carried out for the heat-resistant steels referred to the API-530 standard by applying a model for carbon diffusion with the concurrent formation of alloy carbides. The calculated carbon and carbide volume fraction profiles were validated experimentally. The carburization layer is composed from M₂₃C₆ and M₇C₃ carbides. The time required for the carburization front to reach the mid-thickness of the tubes was used to characterize carburization resistance. The austenitic grades exhibit a higher carburization resistance than the ferritic grades at all temperatures. In the ferritic grades, alloy composition has a stronger effect at lower service temperatures (600 °C) where carburization resistance increases with Cr and Mo content. The acceleration of diffusion at high temperatures (800 °C) dominates the composition effects on carbon diffusion, and the carburization front is controlled by the formation of carbides, which in turn depends on the available amount of Cr in the steel. In the austenitic grades, the highest carburization resistance is exhibited by the stabilized grades 321 and 347 due to formation of TiC or NbC carbides respectively. Regarding the non-stabilized grades, carburization resistance is raised by addition of Mo (316 vs 304) and lower carbon (316L vs 316). The results of this study can be used for material selection for carburization resistance and for planning maintenance procedures for the timely replacement of tubes.     

Fatigue properties of stainless sheet steels in air at room temperature

Abstract

The number of stainless sheet steel applications involving cyclic loading in non-corrosive and mild corrosive environments is increasing. In the present work, the fatigue properties of two austenitic (AISI 301 and AISI 304) and two duplex (Avesta 2205 and SAF 2304) stainless steels have been studied using fully reversed strain controlled testing of smooth specimens in air and at room temperature. Since prestraining is a common operation for increasing the proof stress of austenitic grades, these grades were also tested in the prestrained condition. The fatigue strength has been evaluated as a function of total strain amplitude, plastic strain amplitude, stress amplitude, and using the Neuber parameter.

MST/3155     

Strain hardening features of metastable austenite-martensite steels

A physical approach is developed for describing deformation diagrams of the metastable states of austenite-martensite steels over a wide range of change in the original quantitative ratio of the phases and type of stressed state with biaxial tension. The approach is based on structural effects revealed by experiment of the reciprocal influence of the phases connected with a intentional change in their fine structure, and with development of residual tensile stresses in phase-hardened austenite and its strengthening effect on strain-induced martensite. From the standpoint of the different relationships obtained as applied to steel 07Kh16N6 the different sensitivity of yield and ultimate strengths to the quantitative phase ratio and invariance of generalized deformation curves to the principal stress ratio is substantiated.From papers at the III All-Union symposium Strength of materials and structural elements with a complex stressed state, Zhitomir (1989).Translated from Problemy Prochnosti, No. 6, pp. 23–27, June, 1991.     

Modelling solid solution hardening in stainless steels

The solid solution hardening of stainless steels is studied by using the Labusch–Nabarro relation. Models are evaluated in order to predict the mechanical properties from chemical composition, solution hardening misfit parameters, grain size, ferrite content and product thickness. A data source of six grades of steels is used for the modelling. Both austenitic and duplex stainless steels are covered including more than 1100 batches, which are subjected to multiple regression analyses. The models are compared with earlier studies and can be used as tools in material optimisation.     

Mechanism of surface hardening of structural carbon steels

Results of fatigue tests of steel specimens with different carbon contents are presented. We propose a dislocation model of strength of a two-phase plastically deformed body, according to which the formula for the assessment of the efficiency of hardening and the increase in the fatigue resistance of carbon steels is corrected.     

Oriented thermomechanical hardening and recrystallization of high-carbon steels

Summary Recrystallization of austenite in high-carbon steels after light ( = 20–25%) deformation is not a uniform process because of the nonuniform and localized character of the deformation of austenite grains. The largest increase in the average grain size of austenite in steel ShKh15 takes place during the first 1.5 min of post-deformation annealing, and the grain growth rate is much faster in the transverse than in the longitudinal direction.As a result of recrystallization processes during HTMT of high-carbon steels, their strength and ductility are sometimes reduced below the level recorded for quench-hardened and tempered specimens.Recrystallization during HTMT leads to a reduction in the degree of anisotropy of mechanical properties determined in tests under direct and inverse loads. The results of mechanical tests show, however, that oriented inclusions and a striated structure confer a considerable degree of anisotropy on the properties of recrystallized specimens.The reduction of quasi-brittle strength of martensite after HTMT (followed by recrystallization) is to a large degree attributable to the oriented growth of recrystallized grains; this relation is described by Eq. (2). The reduction in ductility is associated with the reduced quasi-brittle strength of martensite formed from recrystallized austenite.     

Small-angle neutron scattering of multiphase secondary hardening steels

Alloying secondary hardening steels with Ni and Al allows the precipitation of an intermetallic phase B2-NiAl in addition to the classical secondary carbides precipitation, adding up the advantages of both types of precipitation (Erlach et al. Mater Sci Eng A 429:96, 2006; Erlach et al. Int J Microstruct Mater Prop 3:373, 2008). Small-angle neutron scattering experiments were carried out to analyse the nanometric scale precipitation of a martensitic steel containing a double precipitation of carbides and intermetallic phase. The precipitates size, volume fraction and chemical composition for both carbides and intermetallic phases according to the tempering time were estimated and discussed. In addition, experimental cobalt-free grades containing a single precipitation or a double precipitation were manufactured and analysed. Relationship between measured tensile yield strengths and the nanometre-sized particles are suggested, showing that both populations of precipitates have a relevant impact on the mechanical properties.     

Fatigue crack propagation in steels

From previous investigations of the mechanisms of both fracture and fatigue crack propagation, the static fracture model proposed by Lal and Weiss may be thought as reasonable for describing fatigue crack propagation in metals at both low and intermediate stress intensity factor ranges $\text{ΔK}$. Recent progress in fatigue crack propagation indicates that it is not only possible, but also necessary, to modify this static fracture model. Based on the modified static fracture model, the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$, which is defined as the difference between $\text{ΔK}$ and the fatigue crack propagation threshold value $\text{Δ}{}_{\mathrm{th}}$, is taken as the governing parameter for fatigue crack propagation. Utilising the estimates of the theoretical strengths of metals employed in industry, a new expression for fatigue crack propagation, which may be predicted from the tensile properties of the metals, has been derived. The correlation between the fatigue crack propagation rate and the tensile properties is thus revealed. The new expression fits the test results of fatigue crack propagation of steels below 10^?3 mm/cycle and indicates well the effect of stress ratio on the fatigue crack propagation rate.     

Recrystallisation after hot deformation of two phase stainless steels

Abstract

A series of model stainless steels containing 0, 30, 50, 70 and 100% volume fraction of δ was prepared. In these two phase alloys a duplex structure of equiaxed grains of δ and γ with a uniform distribution was established through hot deformation and heat treatments. The static recrystallisation kinetics were followed in all five alloys after hot torsion at a constant strain rate of 0.7 s¹ in the temperature range 1000 - 1150 ° C applying a constant strain of 0.28. Comparison of the static recrystallisation behaviour of the single and two phase alloys has been carried out at 1050 ° C. The results show that both phases recrystallise after hot deformation. However, a significant retardation of recrystallisation in both δ and γ phases has taken place in the two phase alloys in comparison with the single phase alloys, with the delay being larger for the δ phase. The possible causes of the retardation of recrystallisation of δ and γ phases in the two phase alloys are discussed.     

Fatigue properties of carbon steels chromized by a diffusion treatment under various conditions

A study was carried out on the effect of the variation in a gaseous diffusion chromizing treatment conditions on the fatigue properties of chromized carbon steels tested in the elastic and elasto-plastic strain range in air and certain aggressive media. An explanation is given of the dependence of the fatigue and corrosion-fatigue strength on the thickness of chromized layers and on the method of their preparation.