Showing papers on "TRIP steel published in 1978"

Transformation behavior of TRIP steels

Abstract: True-stress (σ), true-strain (e) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs e) of materials in both conditions have a sigmoidal shape at temperatures above Msσ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below Msσ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below Msσ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above Msσ. Comparison of σ-e andf-e curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above Msσ, but there is also a dynamic “transformation softening” contribution which becomes dominant below Msσ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above Msσ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below Msσ.

The Role of Deformation-Induced Phase Transformations in the Plasticity of Some Iron-Base Alloys

Abstract: Microstructural changes in alloys can be induced by phase transformations. While many phase transformations are thermally activated, some are not. An important example of the nonthermally activated type is the deformation-induced phase transformation. Deformation-induced phase transformations are known to cause unusual changes in the mechanical properties of ferrous and nonferrous alloys. In the past several years it has been shown that this type of transformation can considerably enhance the mechanical properties of high-strength austenitic alloys— these alloys are now known as “TRIP” steels. Useful combinations of toughness, strength, and ductility can be obtained in these steels by control of the composition and the processing. TRIP steels are thermomechanically processed in the austenitic state. During this thermomechanical processing, changes occur in both chemistry and substructure, and these alter the stability of austenite with respect to deformation during subsequent mechanical testing. The present chapter discusses the several compositional, processing, and testing variables that influence this austenite stability. It is shown that the strength, ductility, stress-strain behavior, fracture toughness, fatigue properties, and corrosion resistance of TRIP steels are strongly affected by austenite stability. The considerations involved in designing TRIP Steels, their limitations, and some of the steps that have been taken to overcome these limitations, are reviewed. Recent studies are described in which attempts were made to incorporate the TRIP phenomenon in other classes of steels. These include nonnickel cryogenic steels and low and medium alloy quenched and tempered ultrahigh-strength steels.

Stress Corrosion Cracking and Corrosion Fatigue of Some Surgical Implant Materials in a Physiological Saline Environment

Abstract: The results of slow strain rate tensile tests and concomitant fractography suggest that cast Vitallium and Type 316L stainless steel, which are widely used for surgical implants, are not susceptible to stress corrosion cracking (SCC) in a physiological saline (Tyrode's) solution. Several high strength, high ductility stainless steels (TRIP steels), which are candidates for implant use, also appeared to be immune to SCC. Some combinations of TRIP steel composition and metallurgical condition showed significantly lower elongations to failure in Tyrode's solution than in air, but no fractographic evidence of SCC was obtained. The susceptibilities to corrosion fatigue of cold worked Type 316L and two TRIP steels were assessed by comparing the fatigue lives of smooth specimens tested in argon and Tyrode's solution. All three materials generally showed lower fatigue lives in the saline environment, the degree of life reduction being dependent on electrochemical potential. Although additional research i...