The structure, phase composition, and residual stresses of diffusion boride layers formed by thermal-chemical treatment on the die steel surface

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 2 2 / / z z z x x V f l f l = + µ ⋅ ; 2 2 / / x x x z z V f l f l = + µ ⋅ ; ( ) / 2 z x u τ = χ − χ – reduced displacements recorded during the etching process; 0 z f , 0 x f – strip deflections changes caused by notching, mm; z f , x f – the same deflections, registered when removing layers, mm; 0 z χ , 0 x χ – changes in the unit angle of the strip twist caused by the notch, rad / mm; z χ , x χ – the same changes, registered when removing layers, mm; l z , l x – corresponding lengths of the strips, mm; h – the initial plate (strip) thick- ness, mm; a – the layer thickness removed at the experiment time, mm; ξ – the integration variable. Results and discussions The microhardness, structure, phase composition, and stress state were determined in this study. The metallographic analysis revealed that a diffusion layer of 20 μm thick was formed after boriding at 950 °C (Fig. 6, a ). An increase in temperature up to 1050 °C resulted in a diffusion layer formation up to 105 μm thick (Fig. 6, b ). Boride layers have a teeth-like structure with rounded ends oriented in the direction of boron diffusion. In the upper part of the layer, pores were observed, which appeared due to interaction with ambient air. After TCT in both temperature modes, a transition zone was formed in a dark area under the borides. In this case, after high-temperature treatment, light crystals oriented along the grain boundaries of the base metal were detected. Probably it was carboborides (Fe 3 (B,C)) [17]. The maximum microhardness after treatment at 950 °C was 1250 HV, at 1050 °C was 1880 HV corresponding to Fe 2 B and FeB iron borides (Fig. 7) [1-4]. XRD analysis confirmed the presence of Fe 2 B in the diffusion layer after TCT at 950 °C and FeB after TCT at 1050 °C (Fig. 8). In the first case, a high microhardness gradient was observed between the layer and the base metal reaching 800 HV (Fig. 7, a ). In the second case, the values smoothly decreased towards the base metal, which is more preferable for the products operating under alternating stress conditions (Fig. 7, b ). The second softer Fe 2 B boride and a more developed transition zone with carboborides after boriding at 1050 °C contributed to the smooth microhardness profile. In order to determine the layer’s elemental composition in local areas and separate structural components, the EDS analysis was used (Fig. 9). The results of quantitative analysis are presented in Tables 3 and 4. The low-temperature boriding mode led to the formation of Fe 2 B iron boride with the highest boron content of about 6 % in the upper part of the layer (spectrum 1 and a line spectrum 1 on Fig. 9, a ). The boron content after treatment at 1050 °C increased up to 11 %. In both temperature modes increased chromium content was revealed, which is associated with the scanning at a relatively low accelerating voltage to detect light elements, for example, boron. The tungsten content in the diffusion layer varied in the range from 1 to 13 %, which indicated its inhomogeneous distribution in the layer. Also, light carbide inclusions up to 1 μm were observed in its microstructure (Fig. 9, a ). Fig. 5. To an explanation of the formulas for calculating residual stresses by a mechanical method