Since the beginning of vehicle construction, the majority of steel panels and structure came from cold rolled steel. Cold rolling is a metal working process in which metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. Cold rolling increases the strength and hardness of a metal by introducing defects into the metal’s crystal structure. The process is most often used to decrease the thickness of plate and sheet metal.
Cold rolling has been an effective process, but today’s carmakers are focusing on alternative materials for construction — and refocusing on altering the steel itself. A newer process, called hot stamping or press hardening, is the process of forming metal while it is at temperatures in excess of 1,650°F, and then cooling it quickly in the die. The process converts low-tensile-strength metal to a high-strength steel that weighs in at 150 to 200 kilopounds per square inch (KSI).
Baking in strength
A press-hardenable material, primarily boron steel or aluminized steel, is heated to more than 1,650°F in an oven in the first stage of the press line. The material is transferred quickly to a press, where the part is formed while the material is still very hot. Then the part is rapidly cooled by being held in a water-cooled die cavity for a few seconds at the bottom of the press stroke.
The hot stamping process sounds pretty simple: Heat up a steel sheet until it is red-hot, stamp it into a form, and keep it there for a few seconds while it cools. However, in the case of modern hot stamping, relatively complex formed and angled parts must be formed in a single-step stamping process. What results is a fairly intricate part that is much stronger and lighter than the base material. For example, boron steel, in its original state, has tensile strength of around 50 KSI — but after it is hot formed, its tensile strength is about 200 KSI.
Complexity can simplify
Because hot stamping allows the forming of complex parts with a single stamp stroke, multi-component assemblies can be formed as one component, eliminating some subsequent joining processes such as welding. However, new techniques are required for the repair of ultra-high strength hot-formed steel. Straightening and reshaping of even minor deformations are not possible, because of the high-tensile yield strength and the rebound effect of hot-formed steel. A damaged component must be completely cut out and/or partially replaced in accordance with the manufacturer’s specifications.
The use of butt-welding to repair ultra-high-strength hot-formed steel can cause weakness in the repair, because of micro-structural changes arising from heating the steel during the welding process. To help ensure optimum repair integrity, most vehicle makers have service solutions incorporated during the development of the body structure that allow affected components to be replaced wholly or partially in special parting sections.
Hot-formed steel is being used by many carmakers on entry-level vehicles as a way of saving weight and strengthening critical components such as A and B pillars. That means it is critical in the repair estimating process that this type of steel is identified, to carry out a proper repair. The use of traditional cutout and butt-welding repair will weaken the structure and throw off air bag timing, with potentially fatal consequences. Knowing the manufacturer’s repair procedures has always been important, but now it can mean the difference between life and death.
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