Microstructure refinement and its effect on properties of spring steel

The effect of electroslag remelting (ESR) with CaF2-based synthetic slag on the microstructure and mechanical properties of three as-quenched martensitic/martensitic-bainitic ultrahigh-strength steels with tensile strengths in the range of 1250–2000 MPa was investigated. Ingots were produced both without ESR, using induction furnace melting and casting, and with subsequent ESR. The cast ingots were forged at temperatures between 1100 and 950 °C and air cooled. Final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, X-ray diffraction, color etching, and micro-hardness measurements. Mechanical properties were investigated through measurement of hardness, tensile properties and Charpy-V impact toughness. The microstructures of the investigated steels were mainly auto-tempered martensite in addition to small fractions of retained austenite and bainite. Due to the consequences of subtle modifications in chemical composition, ESR had a considerable impact on the final microstructural features: Prior austenite grain, effective martensite grain, and lath sizes were refined by up to 52%, 38%, and 28%, respectively. Moreover, the 95th percentiles in the cumulative size distribution of the precipitates decreased by up to 18%. However, ESR had little, if any, the effect on microsegregation. The variable effects of ESR on mechanical properties and how they depend on the initial steel composition are discussed.

The effect of the deep cryogenic treatment on the micro‐structure and mechanical properties (tensile strength, toughness, residual stress and fatigue strength) of the medium carbon spring steel, which is subjected to different heat treatment steps, is investigated. Deep cryogenic treatment causes spring steel to keep compressive residual stress more efficiently due to an increase in the density of the crystalline defects, retardation in the stress relief after the phase transformations and nano‐cluster carbide formations. If deep cryogenic treatment is applied before the tempering then the homogeneously distributed fine carbides form after the tempering and the grains remain relatively fine. The microstructure with homogeneously distributed fine carbides and fine grains cause spring steels to have simultaneously enhanced tensile strength, ductility and fatigue strength. If deep cryogenic treatment is applied after the conventional heat treatment (quenching+tempering), however, the coarse carbides form in the micro‐structure and the improvement in the mechanical properties of the spring steel is limited.

The purity of a steel is an important factor influencing the quality of the final products. Therefore, it is important to optimize the existing and develop new steelmaking technologies that affect the resulting purity. Electro slag remelting is a technology of tertiary metallurgy, which can advantageously be used to fabricate high quality steels. The study presents selected theoretical aspects of oxide systems and their specific influences on effectiveness of the electro slag remelting technology. The aim of this work was to experimentally analyze the purity of a tool steel fabricated by electro slag remelting using two different oxide systems (fused slags). The core of the study is the determination of the overall presence of elements in the steels, a thorough investigation of the presence of (not only) oxide-based inclusions within the investigated tool steel, and a detailed analysis of their chemical composition, including the size of these non-metallic inclusions, using energy dispersive X-ray (EDX) on the scanning electron microscope (SEM). Last but not least, the determination of the modification of the occurring non-metallic inclusions and verification of the experimentally acquired results as well as the calculation of the liquid and solid temperature and the calculation of the viscosity of the slags using the FactSage calculation software was performed. The results showed that the used slag influenced especially the occurrence of Mg and Al-based oxide inclusions. The CaS-type inclusions were present within all of the examined samples. The slag type influenced not only the typical morphology and size of the inclusions (especially of the CaS type), but also the tendency of the steel to exhibit localized corrosion when exposed to the ambient environment. This research can contribute to a better understanding of the effect of oxidation systems on the resulting purity and properties of ESR steels, thereby advancing the production of tool steels with higher quality and performance requirements.

<div class="section abstract"><div class="htmlview paragraph">15-5PH is a precipitation-hardening, martensitic stainless steel used for primary structural elements such as engine mounts where corrosion resistance, high strength, good fatigue and fracture toughness is required. The material composition is defined in AMS5659M. This alloy can be either Type 1 - vacuum arc remelt (VAR) or Type 2 - electro slag remelt (ESR) and is most commonly heat treated per SAE AMS-H-6875 or AMS2759/3 to condition H1025 (an ultimate tensile strength of 155 ksi [1070 MPa] minimum). Typically material handbooks have limited fatigue data and most data is only for Type 1. Therefore, the fatigue properties of 15-5PH H1025 stainless steel for both Type 1 and Type 2 were determined. The objective of the fatigue testing was to generate a family of S-N curves (maximum stress versus number of cycles to failure) for a series of stress ratios across the entire range of cycles to failure. All test coupons have been made from 5/8″ to 1″ [16-25 mm] diameter bar material with a typical lathe turned surface finish of 40-80 micro-inches [1-2 micro-meters] or a polished surface of 8 micro-inches [0.2 micro-meters]. The machined surface versus the polished surface resulted in a 10% reduction in fatigue strength near the endurance limit. When comparing the family of curves of Type 1 and Type 2 materials no significant differences were found. Therefore, the VAR and ESR Type materials appear to have approximately equivalent fatigue properties based on this set of fatigue data.</div></div>

Cleanliness improvement and microstructure refinement of ingot processed by vacuum electroslag remelting

Role of severe plastic deformation on the cyclic reversibility of a Ti50.3Ni33.7Pd16 high temperature shape memory alloy

This work examines the effect of grain refinement via accumulative roll bonding (ARB) on the homogeneity of microstructure and hardness through the thickness of commercially pure aluminum AA1100 sheets. The use of 7 cycles of ARB was shown to produce homogeneous ultrafine-grained microstructure, reducing the grain size to 0.46 and 1.03 µm in normal direction and rolling direction, respectively. Examination by electron backscattered diffraction indicates that continuous dynamic recrystallization was the main mechanism for the formation of submicron equiaxed grains bounded with high-angle grain boundaries (HAGBs). The fraction of HAGBs increased gradually reaching a maximum of 71.6% after 7 cycles. Through-thickness hardness measurements using Vickers and nanoindentation tests show an increase from 43.5 Hv (0.85 GPa) for as-received sample to 63 Hv (1.1 GPa) after 7 cycles. The heterogeneity in hardness through the thickness of the as-received material was shown to restrain the evolution of uniform hardness across the sheet thickness with increasing ARB cycles. Tensile tests showed that the tensile strength is increased to 250 MPa, which is about 2 times its initial value. Owing to the high stacking fault energy of AA1100, strong dynamic recovery occurred with increasing ARB strain which was balanced with the strain hardening property of the material. This resulted in plastic instability at small strains and thus early necking during the tensile test.

In order to produce extremely high strength values without impairing the impact properties, the microstructures of two low-carbon microalloy steels were modified substantially by carefully designed combinations of hot working and heat treatment. The interplay of several microstructural processes in unconventional variants of thermomechanical treatment was investigated systematically. The experiments were carried out in plane-strain compression using the computer controlled hot-deformation simulator ‘WUMSI’. The results indicate that for optimizing mechanical properties the variants found most suitable were those which lead to a fine microstructure by quenching or which take advantage of grain refining caused by additional reaustenitizing. Furthermore, the controlled superposition of age hardening and subsequent deformation brought about increasing yield and tensile strengths together with some reduction in shelf energy. The beneficial effect of microstructure refinement was more pronounced in the pearlite-reduced Ti steel, whereas the straining of an age-hardened microstructure could be exploited extensively in the acicular-ferritic Nb–Mo steel.MST/55

Influence of strain rate on the tensile properties of metastable austenitic stainless CrNi and CrMnNi spring steels

The present study was undertaken to investigate the effect of thermo-mechanical treatment (TMT) on the microstructure and mechanical behaviour of low alloy steel. Hot forging is carried out at 1200°C using mechanical press of 500 and 800 ton. The effect of hot forging reduction ratios (1.11 and 1.29) on the hardness and mechanical properties are studied. TMT samples are given different heat treatment i.e. annealing (A), normalizing (N), hardening (H), hardening and tempering (H/T) and their corresponding impact toughness are obtained. Selected heat treatment (normalizing and annealing) are given to tensile test samples and their corresponding strength and ductility are obtained. Ultimate tensile, 0.2% offset yield strength and percent elongation are measured. Hardness and impact toughness measurements were carried out for all alloy conditions. Hardness (HV), ultimate tensile stress (UTS-MPa) and 0.2% offset yield stress (MPa) increases with increasing reduction ratio. TMT leads to a sharp rise in alloy hardness and strength. Normalizing and annealing following TMT revealed a low hardness values compared to those observed in the TMT condition. Annealing reduces hardness and strength but increases ductility and impact toughness. This could be attributed to the recovery and coarsening effect. Pro-eutectoid ferrite phase are observed along the grain boundaries of low alloy steel in the TMT conditions regardless of the reduction ratios. Normalized samples show a refined pearlitic microstructure while coarse pearlite is observed in the annealed one. Good mechanical properties can be obtained by a combination of plastic deformation and thermal treatment. Heat treatment is one of the major factors used to enhance the mechanical properties of low alloy steel. An understanding of the combined effect of TMT and subsequent heat treatment on the structure and mechanical properties of low alloy steel would help in selecting conditions required to achieve the optimum mechanical properties and alloy high strength to weight ratio. This may be achieved by measuring hardness, impact toughness, strength and ductility resulting from different heat treatment following TMT.

To develop higher strength spring steels with better performance for advanced automotive and transportation industries in line with the trend of lightweight, the strengthening mechanism of two practical 55SiCr and 55SiCrNbV spring steels, which represents plain and micro alloyed spring steels, has been investigated based on the systematic characterization of microstructures, hardness, and tensile test. The tensile strength and hardness were found to be correlated well in a linear relationship. Then the strengthening coefficients of C, Si, Cr and Mn were determined to quantify the solid solution strengthening. The strengthening mechanism of spring steels was found to be the summation of solid solution strengthening, grain refinement, dislocation, and precipitation strengthening, which agree well with the tensile test results. The strengthening mechanism could be used as guidance to develop next generation spring steels, which well explained the ultra-high strength spring steels over 2200 MPa class in the literature.

Toward the end of the second millennium martensitic steels in the C r N i M o V system (with 12% Cr) have found wide use in power engineering. Such steels possess high strength and ductility, but the problem of simultaneous elevation of the long-term strength and the resistance to brittle fracture still awaits solution. In order to solve this problem we need a technology that provides a high purity of the metal with respect to nonmetallic nonferrous metal inclusions. We have studied the effect of various methods of refining (out-of-furnace refining and degassing, electroslag remelting, vacuum arc remelting) on the properties of high-chromium steel used for the production of large gas turbine discs. It has been established that when the steel bears 872MPa, cyr_>1030MPa, 6>17%, ~g >_60%, KCV>_ 1130 kJ/m 2, and long-term strength cr 50010,000 -400 MPa for 10,000 h at 500~

Ultra-high strength and high ductility 60Si2CrVNb spring steel with multiphase microstructure controlled by austempering

The use of heat straightening to repair damaged steel structures has gained popularity in recent years. However, applications have been limited because of concerns related to degradation of material properties after repair. Most research has been limited to small undamaged plate specimens where only one to three heats were applied. The purpose of this paper is to report on a study in which structural members were damaged and completely repaired by heat straightening, after which material properties were investigated. The research data indicate that heat straightening does affect mechanical properties of steel. Yield stress may increase by as much as 20%, especially in the vicinity of the apex of vee heats. Tensile strength also increases but at only half the rate of yield stress. The ductility as measured by percent elongation may decrease by one-third, and the modulus of elasticity may decrease by over 25% in some heated regions. Damage with maximum strains up to 100 times the yield strain were repaired, and material properties were compared to members damaged with much smaller strains. The degree of damage had a minimal effect on the material properties of heat-straightened steel. Some members were damaged and completely heat straightened more than once to evaluate the effect of repetitive damage on the material properties of heat-straightened steel. Changes in material properties were small after two cycles of damage and repair. However, additional cycles produced a more brittle material and, in some cases, resulted in fracture of the material. In summary, heat straightening is a viable alternative for the repair of damaged structural steel. However, the user should be aware that some material properties will be changed during the process.

Effects of titanium content on the impact wear properties of high-strength low-alloy steels