Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: P
- PDF / 876,930 Bytes
- 9 Pages / 630 x 792 pts Page_size
- 13 Downloads / 235 Views
INTRODUCTION
D E F O R M A T I O N - i n d u c e d transformation of austenite to martensite was first studied in austenitic stainless steels and other highly alloyed steels. [1-14] The transformation may be "stress-induced" or ~strain-induced." The former transformation mechanism is assisted by stress during the initial stages of deformation, while the latter mechanism requires substantial amounts of strain to initiate and sustain the transformation. Deformation-induced transformation increases strain-hardening rates and, in the case of austenitic stainless steels, in limited temperature ranges, produces stress-strain curves with sigmoidal shapes. The presence of inflection points on these curves indicates the formation of significant amounts of martensite from austenite. The deformation-induced transformation of austenite has been shown to increase toughness ]13,14] and tensile ductility. [4-9] Critical to these improvements are the thermal and mechanical stability of the austenite. At low temperatures or in alloys which have austenite of low stability, the deformation-induced transformations occur in early stages of deformation. As a result, there is little benefit of the strain hardening related to deterring plastic instability or necking in the later changes of deformation. At high temperatures, the austenite transforms at higher strains, and the associated strain hardening effectively increases resistance to necking and fracture. At even higher temperatures, i.e., above M d temperatures, no strain-induced transformation occurs, and ductility is reduced. Deformation-induced transformation of retained austenite in ferrite-martensite dual-phase s t e e l s [15-281 is alSO YASUHARU SAKUMA, Visiting Scientist with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is Senior Researcher, Nippon Steel Corporation, Chiba-Ken 299-12, Japan. DAVID K. MATLOCK, Charles F. Fogarty Professor, and GEORGE KRAUSS, Professor, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted May 17, 1991. METALLURGICAL TRANSACTIONS A
expected to enhance strength and ductility, [29-32] but the reported results are controversial. Some authors report little or no benefit of retained austenite.[33.341 The quantity of retained austenite may be too low or the austenite may transform too early during testing to enhance ductility. Retained austenite content in intercritically annealed steels may be enhanced by subcritical isothermal holding. Isothermal holding also produces bainite in low- and medium-carbon steels.t35-42] The room-temperature ductility of isothermally transformed specimens is a function of isothermal holding time and depends on the mixture of martensite, bainite, and retained austenite dispersed in the ferrite matrix retained during intercritical annealing. Part I of this study described the microstructures and room-temperature mechanical properties of two steels intercritically annealed and isothermally transformed, t
Data Loading...
