AIT Welder 2nd Period Practice Test 2026 – Complete Exam Prep Guide

Very high carbon steels have so much carbon that they become extremely hard and brittle when heat-treated. Welding introduces a large, localized heat input and then rapid cooling, which drives the surrounding metal into hard, brittle structures like martensite in the heat-affected zone. That brittle microstructure, along with the thermal stresses from the welding, makes the joint prone to cracking and loss of toughness under service loads. Because of this, welding these steels often results in a weld that is weaker than the surrounding material, so joining them by welding is seldom chosen. If joining is absolutely necessary, it requires careful preheating, controlled heat input, and sometimes post-weld heat treatment, which many applications don’t justify compared to alternative methods. The idea that they aren’t steel isn’t correct—they are steel, but their high carbon content makes welding problematic.