In Situ Investigation of the Evolution of Lattice Strain and Stresses in Austenite and Martensite During Quenching and T
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INTRODUCTION
MODERN steels are multi-phase materials. The response of a multi-phase material to an applied load is a function of the volume fraction, distribution, orientation, and shape of the phases present,[1,2] as well as of the presence and magnitude of internal stresses, which remain after processing.[3] On loading, the applied external forces are superimposed on the internal stresses.[1,2] Internal stresses can be classified by the length scale over which they equilibrate.[1,4,5] Macro-stresses (type I) act over large distances and are an average over all phases and grains present; micro-stresses vary from grain to grain and from phase to phase (type II) or within a single grain/phase (type III). Internal stress can arise as a consequence of inhomogeneous elastic and thermal properties,[2,3] inhomogeneous plastic strain,[2] or a phase transition occurring in association with a shape change.[5]
M. VILLA and M.A.J. SOMERS are with the Department of Mechanical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark. F. NIESSEN is with the Danish Hydrocarbon Research and Technology Centre, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark. Contact e-mail: [email protected] Manuscript submitted February 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The austenite (c)-to-martensite (a¢) transformation in steel is associated with a shape change, the so-called transformation strain, which consists of a volume expansion of approx. 3 pct and a shear.[6–9] Additionally, austenite and martensite have different thermal and elastic properties. Thus, martensite formation is associated with the development of residual stress in the material, with contributions from the transformation itself as well as thermal mismatch.[3,9] The development of macro-stresses during quenching of steel parts is a well-described subject.[3–5,9,10] Similarly, the generation of lattice defects (micro-stresses of type III) in austenite during martensite formation has been investigated in details.[6–8] On the other hand, the evolution of micro-stresses of type II is controversial: martensite formation has been reported to invoke compressive stress,[11–22] tensile stress,[22,23] or no stress[24–27] in austenite, while information about the stress state in the developing martensite is incomplete. The evolution of micro-stresses of type II during martensite formation is of fundamental interest because these stresses affect the transformation kinetics.[28] In the absence of macro-stresses (type I), the grain- or phase-specific micro-stresses (type II) can be evaluated from the lattice strain as experimentally determined by X-ray diffraction (XRD)[1,4] by measuring in the direchkl tion i the lattice spacing, di u ; for a given family of lattice planes, {hkl}, in the crystalline phase u. Comparison of the measured lattice spacing with a reference hkl hkl lattice spacing, dref u ; provides the lattice strain, ei u :
hklu
hkl ei u
¼
hkl
dref u
di
hkl
dref u
:
½1
hkl
The hkl-specific strain ei u represents the average
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