Failure analysis of heat treated HSLA wheel bolt steels

Abstract

Purpose -The aims of this study is to analyze failure of two types of high-strength low-alloy (HSLA) steels which are used in wheel bolts 10.9 grade, boron steel and chromium-molybdenum steel, before and after heat treatment. Design/methodology/approach -The optimum heat treatment to obtain the best tensile behavior was determined and Charpy impact and Rockwell hardness tests were performed on the two steel types before and after the optimum heat treating. Findings -Fractographic studies show a ductile fracture for heat-treated boron steel while indicate a semi-brittle fracture for heat-treated chromium-molybdenum steel. Formation of a small boron carbide amount during heat treating of boron steel results in increment the bolt's tensile strength while the ductility did not changed significantly. In the other hand, formation of chromium and molybdenum carbides during heat treating of chromium-molybdenum steel increased the bolt's tensile strength with a considerable reduction in the final ductility. Originality/value -This paper evaluates failure analysis of HSLA wheel bolt steels and compares their microstructure before and after the loading regime.1. Introduction AISI 15B37 and AISI 4135 steels are of medium carbon high-strength low-alloy (HSLA) steels which are generally quenched and tempered after production (Hall, 2001). The most important alloying elements are chromium and molybdenum in AISI 4135 and boron in AISI 15B37 steels. Chromium and molybdenum displace time-temperature-transformation (TTT) diagrams to the right and limit the austenite single-phase region in iron-iron carbide phase diagram. Boron additives less than 0.003 wt% refine the grains without significant effect on TTT diagrams and cause high strength and hard steels (Avner, 1974;Tomita, 1990). Chromium-molybdenum steels (41xx series) have relatively low cost, good weld ability and ductility. These steels are common ones to produce super-safe bolts. In general, these bolt steels are used in automotive industry, as connectors of bridges parts, in frictional instructions and aerospace applications (Avner, 1974;Xue et al., 2003). For special applications, heat treatment of the bolts is necessary. The austenite transformation type, mechanical properties and final microstructure of bolts are strongly influenced by the dissolved alloying elements in parent austenite (Golozar, 1999). Chromium-molybdenum and boron steel bolts which are produced by cold forging in automobiles are utilized in several parts such as cylinder head bolt (12.9 grade), wheel bolt (10.9 grade), stud bolts and, etc. (Fastener's Handbook). Wheel bolts commonly fail under tensile stress. Therefore, wheel bolts not only must have an excellent tensile stress but also a rather good toughness to prevent catastrophic fracture.

Estudo comparativo entre o aço SAE 1566 e o aço SAE 15B36 aplicados na produção de lâminas utilizadas na agricultura

“…Dubinskii [33] encontraram microestrutura martensítica em aço equivalente ao SAE 1566 após têmpera em óleo e Ali Nazari e Shadi Riahi [34] encontraram essa mesma microestrutura no aço SAE 15B36 também temperado em óleo.…”