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

Benefits can be gained by peering one layer deeper into the grain structure that governs stainless steel's mechanical behavior. Getty Images

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.

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.

When a force acts upon the metal alloy, energy is added to the system. If enough energy is added to cause plastic deformation, the crystal lattices are strained and new dislocations form. It may seem logical that this should increase ductility, because it frees up more spaces and, therefore, creates more potential for dislocation movement. However, when dislocations collide, they can pin each other in place.

As the number and concentration of dislocations increase, more and more dislocations get pinned together, reducing ductility. Eventually there will be so many dislocations that no more cold-work forming can occur. Because the existing pinned dislocations can no longer move, the atomic bonds in the lattice stretch until they break, or fracture. This is why metal alloys work-harden and why there is a limit to the amount of plastic deformation a metal can take before it fractures.

Grains also play a major part in annealing. Annealing a work-hardened material essentially resets the microstructure so ductility can be recovered. During annealing, grains undergo a transformation in three steps:

Imagine a person moving through a crowded train car. Pushing through the crowd is possible only by creating a gap between rows of people, much like a dislocation in a crystal lattice. As they advance, the people behind them fill into the gaps they’ve left while they create new space ahead. Once they reach the other end of the train car, the arrangement of the passengers will have changed. If too many people try to push through at once, the passengers trying to make space to accommodate their movement will bump into each other, and into the walls of the train car, pinning everyone in place. The more dislocations present, the more difficult it becomes for them to move simultaneously.

It is important to understand that a minimal level of deformation is necessary to trigger recrystallization. However, if the metal does not have enough stored deformation energy before it is heated, recrystallization will not occur, and the grains will just continue to grow beyond their original size.

Mechanical properties can be tuned by controlling grain growth. Grain boundaries are essentially a wall of dislocations. They hinder movement.

If grain growth is limited, a higher number of small grains will result. In terms of the grain structure, these smaller grains are considered finer. More grain boundaries mean less dislocation movement and higher strength.

If the grain growth is less limited, the grain structure becomes coarse, with larger grains, fewer boundaries, and lower strength.

Grain size is often referenced as a unitless number, between about 5 and 15. This is a relative scale, related to average grain diameter. The higher the number, the finer the grain size.

The methodology for measuring and rating grain size is outlined in ASTM E112. It involves counting the number of grains in a given area. This is often accomplished by cutting a cross section of the raw material, grinding and polishing it, and etching it with acid to reveal the grains. The count is performed on a microscope under a magnification that allows for an adequate sampling of grains. Assigning an ASTM grain size number suggests a reasonable level of homogeneity in grain shape and diameter. It may even be advantageous to limit the variation in grain size to two or three points to ensure consistent properties throughout the workpiece.

In the case of work hardening, strength and ductility have an inverse relationship. The relationship between ASTM grain size and strength is often positive and strong, and generally the percentage of elongation and ASTM grain size have an inverse relationship. However, excessive grain growth can result in "dead soft" material that can no longer work-harden effectively.

Grain size is often referenced as a unitless number, between about 5 and 15. This is a relative scale, related to average grain diameter. The higher the ASTM grain size value, the more grains per unit area.

The grain size of an annealed material varies with time at temperature and cooling rate. Annealing typically is performed between an alloy's recrystallization temperature and melting point. The recommended annealing range for the austenitic stainless steel alloy 301 is between 1,900 and 2,050 degrees F. It will begin to melt around 2,550 degrees F. By contrast, commercially pure grade 1 titanium should be annealed at 1,292 degrees F and melts around 3,000 degrees F.

During annealing, the recovery and recrystallization processes compete with each other until the recrystallized grains consume all the deformed grains. Recrystallization rate scales with temperature. Once recrystallization is complete, grain growth takes over. A 301 stainless steel workpiece annealed at 1,900 degrees F for an hour will have a finer grain structure than the same workpiece annealed at 2,000 degrees F for the same amount of time.

If the material is not held within the proper annealing range long enough, the resulting structure may be a combination of old and new grains. If uniform properties throughout the metal are desired, the annealing process should be aimed at achieving a uniform and equiaxed grain structure. Uniform means that all the grains are roughly the same size, and equiaxed means that they are all roughly the same shape.

To achieve a uniform and equiaxed microstructure, every workpiece should be exposed to the same amount of heat for the same amount of time and should cool at the same rate. With batch annealing, this is not always easy or possible, so it is important to at least wait until the entire workpiece is saturated at the proper temperature before counting the soak time. A longer soak time and higher temperature will result in a coarser grain structure/softer material and vice versa.

If grain size and strength are related, and the strength is already known, why bother counting grains, right? All destructive tests have variability. Tensile testing, especially at lower thicknesses, is heavily dependent on sample preparation. Premature fractures can occur in tensile strength results that are not representative of the actual material properties.

If the properties are not uniform throughout the workpiece, taking a tensile coupon, or sample, from one edge might not tell the whole story. Sample preparation and testing can also be time-consuming. How many tests, and in how many directions, is it feasible to perform for a given metal? Evaluating the grain structure is extra insurance against surprises.

Anisotropy, Isotropy. Anisotropy refers to the directionality of mechanical properties. Beyond strength, anisotropy can be better understood by examining the grain structure.

A uniform and equiaxed grain structure should be isotropic, meaning that it has the same properties in every direction. Isotropy is especially important in deep-drawing processes in which concentricity is critical. As the blank is drawn into the die, anisotropic material will not flow uniformly, which can result in a defect called earing. Earing occurs where the upper section of the cup develops a wavy profile. Inspecting the grain structure can reveal where the nonuniformities are in the workpiece and help diagnose the root cause.

Proper annealing is essential in achieving isotropy, but it is also important to understand the level of deformation before annealing. As material is plastically deformed, the grains begin to distort. In the case of cold rolling, where thickness is converted to length, the grains will elongate in the rolling direction. As the aspect ratios of the grains change, so will the isotropy and bulk mechanical properties. In the case of a severely deformed workpiece, some of the directionality may be retained even after annealing. This results in anisotropy. For deep-drawn materials, it is sometimes necessary to limit the amount of deformation prior to final anneal to avoid earing.

Orange Peel. Earing is not the only grain-related deep-drawing defect. Orange peel can occur when drawing raw material with grains that are too coarse. Each grain deforms independently, and as a function of its crystallographic orientation. Differences in deformation between neighboring grains result in a textured appearance that resembles an orange peel. The texture is the grain structure revealing itself on the surface of the cup wall.

Just like pixels in a television screen, the differences of each individual grain will be less apparent with a fine grain structure, effectively increasing the resolution. Specifying mechanical properties alone may not be enough to ensure a fine enough grain size to prevent orange peel effects. When the change in dimensions of a workpiece is less than 10 times the grain diameter, the properties of individual grains will drive forming behavior. Instead of deformation being averaged over many grains, it will reflect the specific size and orientation of each individual grain. This is visible by the orange peel effect on the wall of a drawn cup.

For an ASTM grain size of 8, the average grain diameter is 885 µin. This means any reduction in thickness of 0.00885 in. or less may be influenced by this micro forming effect.

While coarse grains can cause problems for deep drawing, sometimes they are recommended for coining. Coining is a deformation process where a blank is compressed to impart a desired surface topography, such as the profile of George Washington's face on a quarter. Unlike drawing, coining does not usually involve much bulk material flow but does require a great deal of force, which may deform only the surface of the blank.

For this reason, minimizing the flow stress at the surface by using a coarser grain structure can help mitigate the force needed for proper die filling. This is especially applicable in the case of open die coining, where dislocations on surface grains are allowed to flow freely, instead of accumulating at grain boundaries.

The trends discussed here are generalizations that may not apply to a specific part. They do, however, highlight the benefits of measuring and standardizing raw material grain size while designing a new part to avoid common pitfalls and optimize forming parameters.

Precision metal stampers and manufacturers performing deep-draw operations on metal to form their parts would be well served to partner with metallurgists at a technically competent precision reroller who can help them optimize their material down to the grain level. When the metallurgy and engineering experts on both sides of the relationship integrate into a single team, it can have transformative effects and result in more positive outcomes.