Strength is a measure of the stress that a crack-free metal can bear before deforming or breaking under a single applied load. Fracture toughness is a measure of the amount of energy required to fracture a material that contains a crack. The tougher the material, the more energy required to cause a crack to grow to fracture. For a particular alloy, lower fracture toughness corresponds to less ductility. For example, glass has very low toughness and is very brittle.
There are several situations during the life cycle of a product when a design team selects a material to use for a component or a joint between components. In addition to selecting materials during new product development companies also change materials to reduce costs, improve product performance and reliability, and improve manufacturing yields. Regardless of the situation, the goals are the same – find the lowest cost solution that enables the product's performance and reliability. There are several steps to the material selection process. This article discusses the first step – defining the requirements for the material.
Stainless steel is known for its corrosion resistance in many environments in which carbon and low alloy tool steels would corrode. The corrosion resistance is a result of a very thin (about 5 nanometers) oxide layer on the steel's surface. This oxide layer is referred to as a passive layer since it renders the surface electrochemically passive in the presence of corrosive environments. The passive layer forms because of the chromium added to stainless steel.
Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith to buy a sharp, shiny long sword. A few weeks later you’re in a battle, fighting at the front of the shield wall. You take a huge swing at the enemy, who meets your blow with his sword, and your sword shatters into several pieces. Unfortunately for you, your blacksmith outsourced a batch of swords to a blacksmith on the other side of town who didn’t have time to temper the swords. As a result, the swords were strong, but brittle.
Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, the heat treating temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties. Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases. Using a temperature that is too low can result in incomplete metallurgical transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.