Martensitic transformation and stress partitioning in a high-carbon steel Matteo Villa, ⇑ Flemming B. Grumsen, Karen Pantleon and Marcel A.J. Somers Technical University of Denmark, Department of Mechanical Engineering, DK 2800 Kgs. Lyngby, Denmark Received 18 May 2012; revised 18 June 2012; accepted 19 June 2012 Available online 1 July 2012 Martensitic transformation in a high-carbon steel was investigated with (synchrotron) X-ray diffraction at sub-zero Celsius tem- perature. In situ angular X-ray diffraction was applied to: (i) quantitatively determine the fractions of retained austenite and mar- tensite; and (ii) measure the evolution of the lattice strain in retained austenite. Ex situ (synchrotron) energy-dispersive X-ray diffraction was performed to assess the effects of the martensitic transformation on the development of stresses in austenite. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Martensitic phase transformation; X-ray diffraction (XRD); Martensite; Retained austenite; Residual stresses Martensitic transformation followed by appro- priate tempering is the most effective mechanism to ob- tain steel products with high strength. Also, in modern multiphase steels, like advanced high-strength steels, it plays a decisive role in maximizing the mechanical char- acteristics of a steel product. The mechanical behavior of a multiphase material is determined not only by the properties of the phases con- sidered individually, but (mainly) by the mechanical interaction between the phases [1,2]. In recent years, the distribution of stresses, and consequently strains, in a multiphase steel during applied deformation has been widely studied [3,4]. In contrast, almost no work has been devoted to the natural effect of the self-stand- ing transformation promoted during cooling, for exam- ple in the sub-zero Celsius range, despite the fact that cryogenic treatment to improve steel performance has gained popularity in heat treatment practice. The purpose of the present work is to investigate the stress state that is evoked in austenite by the martensitic transformation during a controlled sub-zero Celsius treatment of a partially transformed steel, containing austenite and martensite, after quenching to room temperature. To the authors’ knowledge, only a few contradictory experimental studies regarding the effect of martensite formation on the stress state of austenite have been pub- lished: compression of retained austenite due to martens- itic transformation in steel is reported from in situ synchrotron X-ray diffraction (XRD) experiments [5,6], while the build-up of hydrostatic compression was sug- gested on the basis of conventional laboratory XRD experiments [7–9]. In contradiction, neutron diffraction experiments have indicated that martensite formation has no influence on the stress state in austenite [10]. The alloy used in the present work is a commercial AISI 52100 steel that was extruded to a Ø 10 mm rod. The chemical composition of the material as determined with glow discharge optical emission spectroscopy (GDOES; Horiba Jobin Yvon GD2) is reported in Table 1. Disks of thickness 0.33 mm and diameter 10 mm were austenitized at 1323 K for 15 min, to dissolve all car- bides. After austenitization, samples were quenched in oil and kept for 3 min at an austempering temperature of 413 K to promote temperature homogenization. Thereafter, the samples were air cooled to room temper- ature. The initial microstructure thus obtained is shown in Figure 1a and compared with a sample that was addi- tionally cooled at 0.5 K min 1 to 93 K, held 7 h at 93 K and then reheated to room temperature at 1.5 K min 1 . In situ (synchrotron) angular XRD was performed at MagS station (HZB-BESSY II) [11] with a radiation of wavelength k = 0.1 nm focused onto a 1 mm spot, apply- ing Bragg–Brentano geometry. A quantification of the volume fraction of retained austenite was obtained by considering the 200 c , 220 c and 311 c Bragg peaks of austenite, and the 200/002 a 0 and 211/112 a 0 doublet peaks of martensite (see Ref. [5] for more details). Lattice (macro-) strains in austenite in the direction 1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.06.027 ⇑ Corresponding author. E-mail: matv@mek.dtu.dk Available online at www.sciencedirect.com Scripta Materialia 67 (2012) 621–624 www.elsevier.com/locate/scriptamat