How do titanium, stainless steel grain structures affect part forming?

The selection of stainless steel and aluminum alloys often is centered around strength, ductility, elongation, and hardness. These properties indicate how a metal’s building blocks behave in response to an applied load. They are effective metrics for managing the limits of a raw material; that is to say, how much it will bend before it breaks. The raw material must be able to withstand the forming process without breaking.

Destructive tensile and hardness tests can be a reliable, cost-effective way to determine mechanical properties. However, these tests are not always as reliable once the thickness of the raw material begins to constrain the dimensions of the test specimen. Tensile testing a flat metal product certainly still is useful, but benefits can be gained by peering one layer deeper into the grain structure that governs its mechanical behavior.

What Are Grains, Crystal Structures, and Phases?

Metal consists of an array of microscopic crystals called grains. They are randomly distributed throughout the metal. Atoms of an alloy’s elements, such as iron, chromium, nickel, manganese, silicon, carbon, nitrogen, phosphorus, and sulfur in the case of austenitic stainless steel, are an individual grain’s building blocks. These atoms form a solid solution of metal ions bonded into a lattice by their shared electrons.

An alloy’s chemical composition directs the grains’ thermodynamically preferred repeating arrangement of atoms, called a crystal structure. A homogenous section of metal comprising one repeating crystal structure forms one or more grains called a phase. An alloy’s mechanical properties are a function of crystal structures in the alloy. The size and arrangement of the grains of each phase factor in as well.

How Do Grains Form?

The phases of water are familiar to most. When liquid water freezes, it turns into solid ice. However, when it comes to metals, there is not just one solid phase. Certain alloy families are named after their phases. Within stainless steel, the austenitic 300 series alloys consist mainly of austenite when annealed. However, 400 series alloys consist of either ferrite in 430 stainless or martensite in 410 and 420 stainless steel alloys.

The same goes for titanium alloys. The names of each alloy group indicate their dominant phase at room temperature--either alpha, beta, or a mixture of both. There are alpha, near-alpha, alpha-beta, beta, and near-beta alloys.

When a liquid metal solidifies, solid grains of the thermodynamically preferred phase will precipitate where the pressure, temperature, and chemical composition allow them to. This usually occurs at an interface, like ice crystals do on the surface of a warm pond on a cold day. When a grain nucleates, the crystal structure grows in one orientation until it encounters another grain. Because crystal structures are oriented differently, a grain boundary is formed at the intersection of the mismatched lattices. Imagine dropping a bunch of Rubik’s Cubes of differing sizes in a box. Each cube has the square grid arrangement, but they will all settle in different, random orientations. A fully solidified metal workpiece consists of an array of seemingly randomly oriented grains.

Anytime a grain is formed, there is a chance for line defects to develop. These defects are missing pieces of a crystal structure known as dislocations. These dislocations and their subsequent movement throughout a grain and across grain boundaries are the basis of metal ductility.

A cross section of the workpiece is mounted, ground, polished, and etched to view the grain structure. When uniform and equiaxed, a microstructure viewed on an optical microscope looks somewhat like a jigsaw puzzle. In reality, grains are three-dimensional, and each grain’s cross section will look different depending on the orientation of the work-piece cross section.

When a crystal structure is full of all of its atoms, there is no room for movement beyond the atomic bonds stretching.

When you remove half of a row of atoms, you create an opportunity for another row to slip into that spot, effectively moving the dislocation. When a force acts on a workpiece, the aggregate movement of the dislocations in a microstructure allows for it to bend, stretch, or compress without breaking, or fracture.

How Do Grains Factor Into Mechanical Proper