Q235 (Hot Rolled Condition) Metallographic Analysis
Metallographic analysis is a way to examine the microstructure of a metal object in order to determine its composition and any defects present. This technique is particularly useful for analyzing the strength and ductility of metals, as well as for detecting corrosion, welding defects, and other areas of interest. Q235 steel, a grade within the ASTM A36 series, is a commonly used carbon alloy steel that can be hot rolled into a variety of shapes. In order to better understand the properties of the steel, and later be able to use this information to infer possible defects, a metallographic analysis of the material was conducted.
The steel was first cut to size and lapped to ensure a flat and smooth surface for the analysis. Once the sample was prepared, it was mounted onto an acrylic resin and then embedded in an Epoxyset 808 epoxy. This technique allowed for formation of a thin and uniform coating around the steel, making it see-through when it was polished. After the steel was mounted, it was then polished using silicon-carbide paper, beginning with a coarse grade and slowly moving up to finer grades to obtain an mirror-like finish. The sample was then contrast etched using a 1% hydrochloric acid solution to better highlight the various grains in the steel.
The resulting microstructure consisted of numerous lath shaped grains of ferrite and pearlite, along with smaller grains of proeutectoid cementite. Upon closer examination, a small number of martensite needs were also present at the edges of the lath shaped grains. The ferrite and pearlite grains were arranged in a ferrite-pearlite pattern, with pearlite located along the grain boundaries, while no carbides were observed. The average pearlite spacing between the ferrite and pearlite grains was 2.2 micrometers. Aggregate of the pearlite was characteristic of a warm rolling process.
Further analysis of the material revealed the grain size of the ferrite to be in the medium range, making it both tough and ductile in nature. This is due to the ferrite, which can deform in a ductile manner when subjected to stress, providing the steel with excellent plasticity. The pearlite also helped to improve the strength of the material, as it is a hardened mixture of ferrite and cementite. The presence of very small amounts of alloying elements, such as Mn and Si, were also detected in the material.
Based off the results of the metallographic analysis, the Q235 steel was confirmed to have the properties expected from a hot rolled condition. The grains were found to be uniform in size, exhibiting the typical ferrite-pearlite pattern of a medium strength, ductile material. It was also determined that the steel contains very small amounts of alloying elements, no carbides, and a warm rolling process was used in its production. With this metallographic analysis, the desired goal of understanding the microstructure of the steel was achieved.
