Authors George E. Totten, Maurice A. H. Howes, and Tatsuo Inoue note that one step of properly hardening metal is to increase the plasticity of the material. High-strength steel can be hardened with few problems. However in low-strength steels, such as SAE 1045, there are problems in accomplishing this goal. Normalizing can stable stresses in high-strength steel. However, in soft steel, the residual stress relaxation is greater and there is little influence on macro residual stresses.
The lack of stability in large regions of the soft material can result in localized hardening. This phenomenon is known as work hardening and can result from processes such as milling, grinding or turning that may be necessary to prepare the metal. This localized hardening will decrease the plasticity of the materials in these spot. The effects of the work hardening will be largely determined by the size of the affected area and the presence of cracks to relieve stress. One solution for this problem is relief heat treatment.
Igor Altenberger, Ivan Nikitin, and B. Scholtes of the Institute of Materials Engineering at the University of Kassel note the use of rolling of low-strength steel such SAE 1045. At room temperature, the steel can be rolled and strengthened as a result of hardened layers. At temperatures from 320 to 350 degrees Centigrade, rolling results in significantly increased hardness of the surface layer. However, since this process addresses stresses and results in hardening of only the surface layer, classification as an actual hardening process could be an issue.
Joseph R Davis, author of "Surface Hardening of Steels: Understanding the Basics," notes that surface hardening is a widely used approach that increases the wear characteristics of the exterior of metals. However, surface hardening does not affect the softer center of the material. This approach retains the natural toughness of the interior material. Another issue regarding surface hardening is the introduction of such materials as carbon to achieve the hardening.
One method of surface hardening is induction heating. Electroheat Technologies, LLC notes that this approach is suitable for limited applications and cannot replace other hardening methods.