Steels for bearings

Microstructure based prediction of strain hardening behavior of dual phase steels

Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire

Microstructural aspects upon hydrogen environment embrittlement of various bcc steels

The hot compression behavior of a 17-4 PH stainless steel (AISI 630) has been investigated at temperatures of 950 °C to 1150 °C and strain rates of 10−3 to 10 s−1. Glass powder in the Rastegaev reservoirs of the specimen was used as a lubricant material. A step-by-step procedure for data analysis in the hot compression test was given. The work hardening rate analysis was performed to reveal if dynamic recrystallization (DRX) occurred. Many samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall toward the steady-state stress. At low Zener–Hollomon (Z) parameter, this material showed a new DRX flow behavior, which was called multiple transient steady state (MTSS). At high Z, as a result of adiabatic deformation heating, a drop in flow stress was observed. The general constitutive equations were used to determine the hot working constants of this material. Moreover, after a critical discussion, the deformation activation energy of 17-4 PH stainless steel was determined as 337 kJ/mol.

Flow Curve Analysis of 17-4 PH Stainless Steel under Hot Compression Test

This study was aimed at investigating the effect of the microstructural constituents of high strength steels on their hydrogen delayed fracture properties. For this purpose, a series of constant loading tests, slow strain rate tests, cyclic corrosion tests, and thermal desorption spectrometry analysis was conducted on the hydrogen pre-charged specimens with tempered martensite or full pearlite showing a similar tensile strength level of 1600 MPa. Constant loading tests and slow strain rate tests revealed that the tempered martensitic steel was more susceptible to hydrogen delayed fracture than the fully pearlitic steel. In slow strain rate tests, the maximum tensile strength decreased with increasing diffusible hydrogen content in a power-law manner. The content of hydrogen inflowing from environment was also simulated by cyclic corrosion tests. It was found that the fully pearlitic steel has the higher equilibrium hydrogen content than the tempered martensitic steel. The primary trapping sites were prior austenite grain boundaries for the tempered martensitic steel, and ferrite/cementite interfaces and dislocations for the fully pearlitic steel. SEM fractographs revealed that the cracks induced by hydrogen propagated along the prior austenite grain boundaries resulting in brittle intergranular fracture for the tempered martensitic steel while the fully pearlitic steel was fractured in a ductile manner.

Microstructural influences on hydrogen delayed fracture of high strength steels

Dual-phase steel has received much attention as a candidate for non-heat treated steel due to its high strain hardening capability. Present study was performed to examine the optimum microstructure and corresponding heat treatment path revealing excellent cold formability of drawn dual-phase steel. Microstructures were varied by three different heat treatments (or quenching paths), namely intercritical quenching (IcQ), intermediate quenching (ImQ), and step quenching. Resulting microstructures consisted of ferrite and martensite phases, but the volume fraction and morphology were largely varied by the quenching paths. Tensile and compression tests for all three microstructures showed continuous yielding curves with high strain hardening exponent. However, after imposing some amounts of drawing strain, all the specimens were significantly strengthened losing strain hardening capability. Cold formability was studied by estimating both the forming limit and the deformation resistance. The IcQ microstructure showed excellent forming limit and deformation resistance as compared to other two microstructures. It was attributed to the beneficial morphology of the IcQ microstructure, which effectively suppressed crack propagation.

Effect of heat treatment path on the cold formability of drawn dual-phase steels

The hardenability of low-carbon boron-added steels containing molybdenum or chromium was studied using dilatometry, thermodynamic calculations, and secondary ion mass spectroscopy (SIMS). The combined addition of boron and molybdenum was found to be more effective than that of boron and chromium in enhancing the hardenability of boron-added steels. In particular, the addition of 0.5 wt.% molybdenum to the boron-added steel almost completely suppressed the formation of polygonal ferrite even at a slow cooling rate of 0.5℃/s. The synergistic effect of the combined addition of molybdenum and boron is thought to be due to both the suppression of M23(C,B)6 precipitation resulting from the deterioration of phase stability and the reduction of carbon diffusivity by the molybdenum addition.

Effect of molybdenum and chromium on hardenability of low-carbon boron-added steels

Provided is a steel wire rod for a high strength and high toughness spring having excellent cold workability, the steel wire rod having a composition comprising: in weight %, C: 0.4 to 0.7%, Si: 1.5 to 3.5%, Mn: 0.3 to 1.0%, Cr: 0.01 to 1.5%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0020% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, remainder Fe, and other unavoidable impurities, having a microstructure formed of ferrite and pearlite, and in which a prior (before cooling) austenite grain size is 8 μm or less.

Steel wire rod for high strength and high toughness spring having excellent cold workability, method for producing the same and method for producing spring by using the same

In this study, cracks formed in the edge side of Bi-S–based free-machining steel billets during hot rolling were analyzed in detail, and their formation mechanisms were clarified in relation with microstructure. Particular emphasis was placed on roles of bands of pearlites or C- and Mn-rich regions and complex iron oxides present in the edge side. Pearlite bands in the cracked region were considerably bent to the surface, while those in the noncracked region were parallel to the surface. This was because the alignment direction of pearlite bands was irregularly deviated up to 45 deg from the normal direction parallel to the surface, while the billet was rolled and rotated at 90 deg in the same direction between rolling passes. On the edge side, where pearlite bands were bent, iron oxides intruded deeply into the interior along pearlite bands, which worked as stress concentration sites during hot rolling and, consequently, main causes of the crack initiation in the rolled billet. On the surface of the wire rod rolled from the cracked billet, a few scabs were found when some protrusions were folded during hot rolling. In order to prevent the cracking in billets and scab formation in wire rods, (1) the increase of rolling passes and the decrease of reduction ratio for homogeneous rolling of billets and (2) the reduction in sulfur content for minimizing the formation and intrusion of complex iron oxides were suggested.

Duk Lak Lee

Papers

Microstructural influences on hydrogen delayed fracture of high strength steels

Effect of heat treatment path on the cold formability of drawn dual-phase steels

Effect of molybdenum and chromium on hardenability of low-carbon boron-added steels

Steel wire rod for high strength and high toughness spring having excellent cold workability, method for producing the same and method for producing spring by using the same

Formation Mechanisms of Cracks Formed During Hot Rolling of Free-Machining Steel Billets