J. Phys. IV France 125 (2005) 523-526  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125121 Inspection of an end quenched plain steel Jominy bar with photothermal radiometric techniques Y. Liu, N. Baddour, A. Mandelis and C. Wang Center for Advanced Diffusion Wave Technologies, Department of Mechanical and Industrial Engineering University of Toronto, 5 King’s College Road, Toronto, ON, M5S 3G8, Canada Abstract. The effect of the cooling rate on hardness and thermal conductivity in a metallurgical Jominy bar made of AISI 1018 steel, by means of a water end-quenched heat treatment process without diffusion-controlled case depth, is studied with photothermal radiometry (PTR). It is concluded that our two PTR techniques, common-mode rejection demodulation (CMRD) and conventional 50% duty-cycle square-wave frequency scan, are sensitive to low hardness values and gradients, unlike the high values all previous photothermal studies have dealt with to-date. Both PTR methods have yielded an anti- correlation between thermal conductivity and microhardness in this case as in previous cases with heat- treated and diffusion-controlled case depth profiles. It is shown that the cooling rate strongly affects both hardness and thermal conductivity in the Jominy-bar heat-treating process. 1. INTRODUCTION A number of photothermal applications to hardness measurements in metals have been reported in the literature. Establishing a quantitative correlation between the microhardness and thermal conductivity or thermal diffusivity is a key issue to performing reliable photothermal hardness measurements. Many research groups [1-6] have reported a well-established anti-correlation between thermal diffusivity/thermal conductivity and microhardness. But most investigations to-date have focused on samples heat treated in the presence of conventional elements, such as carbon or nitrogen, to form a concentration gradient which subsequently defines the hardness case depth profile after quenching. The effect of heat treatment on the hardness and conductivity is a combination of specific gas exposure in a furnace and cooling rate. The relationship between hardness and thermal diffusivity in carburized and carbonitrided steels has been investigated [6,7]. However, the type of relationship between hardness and thermophysical properties resulting from the cooling rate of steel still remains unclear. All previous works [1-6] have concluded that photothermal techniques are sensitive to high hardness (>200 HV) and large hardness gradient. The applicability of photothermal methods to low hardness values (< 200 HV) and gradients has not been studied. In this work we used laser infrared photothermal radiometry (PTR) to explore this issue. A Jominy bar [8] made of AISI 1018 steel was end-quenched to form a mild hardness profile with continuously varied cooling rate along the bar with maximum hardness less than 14 HC (approx. 140 HV). Two types of PTR signal techniques were used: conventional 50% duty-cycle square-wave modulation and common-mode rejection demodulation (CMRD) [9]. Vertical spatial scans along the side of the end quenched Jominy bar were performed and local hardness and thermal conductivity were measured and compared. 2. LASER-BEAM MODULATION AND SAMPLE PREPARATION Two kind of excitation laser modulations, two-pulse waveform modulation and conventional square waveform modulation, were applied respectively to CMRD [9] and the conventional square-wave PTR spatial scan. The one dimensional thermal-wave signal response is given by:   eff eff k c k i R I f S / 1 / ) 1 )( 1 ( 0       (1) Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005125121