Benefits can be gained by gaining insight into one layer of the grain structure that controls the mechanical behavior of stainless steel.Getty Images
The selection of stainless steel and aluminum alloys generally centers around strength, ductility, elongation, and hardness.These properties indicate how the building blocks of the metal respond to applied loads.They are an effective indicator of managing raw material constraints; that is, how much it will bend before breaking.The raw material must be able to withstand the molding process without breaking.
Destructive tensile and hardness testing is a reliable, cost-effective method for determining mechanical properties.However, these tests are not always as reliable once the thickness of the raw material begins to limit the size of the test sample.Tensile testing of flat metal products is of course still useful, but benefits can be gained by looking more deeply at one layer of the grain structure that controls its mechanical behavior.
Metals are made up of a series of microscopic crystals called grains.They are randomly distributed throughout the metal.Atoms of alloying elements, such as iron, chromium, nickel, manganese, silicon, carbon, nitrogen, phosphorus and sulfur in austenitic stainless steels, are part of a single grain.These atoms form a solid solution of metal ions, which are bonded into the crystal lattice through their shared electrons.
The chemical composition of the alloy determines the thermodynamically preferred arrangement of atoms in the grains, known as the crystal structure.Homogeneous portions of a metal containing a repeating crystal structure form one or more grains called phases.The mechanical properties of an alloy are a function of the crystal structure in the alloy.The same goes for the size and arrangement of the grains of each phase.
Most people are familiar with the stages of water.When liquid water freezes, it becomes solid ice.However, when it comes to metals, there is not just one solid phase.Certain alloy families are named after their phases.Among stainless steels, austenitic 300 series alloys consist primarily of austenite when annealed.However, 400 series alloys consist of ferrite in 430 stainless steel or martensite in 410 and 420 stainless steel alloys.
The same goes for titanium alloys.The name of each alloy group indicates their predominant phase at room temperature – alpha, beta or a mixture of both.There are alpha, near-alpha, alpha-beta, beta and near-beta alloys.
When the liquid metal solidifies, the solid particles of the thermodynamically preferred phase will precipitate where pressure, temperature and chemical composition allow.This usually happens at interfaces, like ice crystals on the surface of a warm pond on a cold day.When grains nucleate, the crystal structure grows in one direction until another grain is encountered.Grain boundaries form at the intersections of mismatched lattices due to the different orientations of the crystal structures.Imagine putting a bunch of Rubik’s cubes of different sizes in a box.Each cube has a square grid arrangement, but they will all be arranged in different random directions.A fully solidified metal workpiece consists of a series of seemingly randomly oriented grains.
Any time a grain is formed, there is a possibility of line defects.These defects are missing parts of the crystal structure called dislocations.These dislocations and their subsequent movement throughout the grain and across grain boundaries are fundamental to metal ductility.
A cross-section of the workpiece is mounted, ground, polished and etched to view the grain structure.When uniform and equiaxed, the microstructures observed on an optical microscope look a bit like a jigsaw puzzle.In reality, the grains are three-dimensional, and the cross-section of each grain will vary depending on the orientation of the workpiece cross-section.
When a crystal structure is filled with all its atoms, there is no room for movement other than the stretching of the atomic bonds.
When you remove half of a row of atoms, you create an opportunity for another row of atoms to slip into that position, effectively moving the dislocation.When a force is applied to the workpiece, the aggregated motion of dislocations in the microstructure enables it to bend, stretch or compress without breaking or breaking.
When a force acts on a metal alloy, the system increases energy.If enough energy is added to cause plastic deformation, the lattice deforms and new dislocations form.It seems logical that this should increase ductility, as it frees up more space and thus creates the potential for more dislocation motion.However, when dislocations collide, they can fix each other.
As the number and concentration of dislocations increase, more and more dislocations are pinned together, reducing ductility.Eventually so many dislocations appear that cold forming is no longer possible.Since existing pinning dislocations can no longer move, the atomic bonds in the lattice stretch until they break or break.This is why metal alloys work harden, and why there is a limit to the amount of plastic deformation a metal can withstand before breaking.
Grain also plays an important role in annealing.Annealing a work-hardened material essentially resets the microstructure and thus restores ductility.During the annealing process, the grains are transformed in three steps:
Imagine a person walking through a crowded train car.Crowds can only be squeezed by leaving gaps between the rows, like dislocations in a lattice.As they progressed, the people behind them filled the void they left, while they created new space in front.Once they reach the other end of the carriage, the arrangement of passengers changes.If too many people try to pass at once, passengers trying to make room for their movement will collide with each other and hit the walls of the train cars, pinning everyone in place.The more dislocations that appear, the harder it is for them to move at the same time.
It is important to understand the minimum level of deformation required to trigger recrystallization.However, if the metal does not have enough deformation energy before being heated, recrystallization will not occur and the grains will simply continue to grow beyond their original size.
Mechanical properties can be tuned by controlling grain growth.A grain boundary is essentially a wall of dislocations.They hinder movement.
If grain growth is restricted, a higher number of small grains will be produced.These smaller grains are considered finer in terms of grain structure.More grain boundaries means less dislocation motion and higher strength.
If grain growth is not restricted, the grain structure becomes coarser, the grains are larger, the boundaries are less, and the strength is lower.
Grain size is often referred to as a unitless number, somewhere between 5 and 15.This is a relative ratio and is related to the average grain diameter.The higher the number, the finer the granularity.
ASTM E112 outlines methods for measuring and evaluating grain size.It involves counting the amount of grain in a given area.This is usually done by cutting a cross-section of the raw material, grinding and polishing it, and then etching it with acid to expose the particles.Counting is performed under a microscope, and the magnification allows adequate sampling of the grains.Assigning ASTM grain size numbers indicates a reasonable level of uniformity in grain shape and diameter.It may even be advantageous to limit variation in grain size to two or three points to ensure consistent performance across the workpiece.
In the case of work hardening, strength and ductility have an inverse relationship.The relationship between ASTM grain size and strength tends to be positive and strong, generally elongation is inversely related to ASTM grain size.However, excessive grain growth can cause “dead soft” materials to no longer work harden effectively.
Grain size is often referred to as a unitless number, somewhere between 5 and 15.This is a relative ratio and is related to the average grain diameter.The higher the ASTM grain size value, the more grains per unit area.
The grain size of the annealed material varies with time, temperature and cooling rate.Annealing is usually performed between the recrystallization temperature and melting point of the alloy.The recommended annealing temperature range for austenitic stainless steel alloy 301 is between 1,900 and 2,050 degrees Fahrenheit.It will start melting around 2,550 degrees Fahrenheit.In contrast, commercially pure grade 1 titanium should be annealed at 1,292 degrees Fahrenheit and melt around 3,000 degrees Fahrenheit.
During annealing, the recovery and recrystallization processes compete with each other until the recrystallized grains consume all deformed grains.The recrystallization rate varies with temperature.Once recrystallization is complete, grain growth takes over.A 301 stainless steel workpiece annealed at 1,900°F for one hour will have a finer grain structure than the same workpiece annealed at 2,000°F for the same time.
If the material is not held in the proper annealing range long enough, the resulting structure may be a combination of old and new grains.If uniform properties are desired throughout the metal, the annealing process should aim to achieve a uniform equiaxed grain structure.Uniform means that all grains are approximately the same size, and equiaxed means that they are approximately the same shape.
To obtain a uniform and equiaxed microstructure, each workpiece should be exposed to the same amount of heat for the same amount of time and should cool at the same rate.This is not always easy or possible with batch annealing, so it is important to at least wait until the entire workpiece is saturated at the appropriate temperature before calculating the soak time.Longer soak times and higher temperatures will result in a coarser grain structure/softer material and vice versa.
If grain size and strength are related, and the strength is known, why calculate grains, right?All destructive tests have variability.Tensile testing, especially at lower thicknesses, is largely dependent on sample preparation.Tensile strength results that do not represent actual material properties may experience premature failure.
If the properties are not uniform throughout the workpiece, taking a tensile test specimen or sample from one edge may not tell the whole story.Sample preparation and testing can also be time-consuming.How many tests are possible for a given metal, and in how many directions is it feasible?Evaluating the grain structure is an extra insurance against surprises.
Anisotropic, isotropic.Anisotropy refers to the directionality of mechanical properties.In addition to strength, anisotropy can be better understood by examining the grain structure.
A uniform and equiaxed grain structure should be isotropic, which means it has the same properties in all directions.Isotropy is especially important in deep drawing processes where concentricity is critical.When the blank is pulled into the mold, the anisotropic material will not flow uniformly, which can lead to a defect called earing.The earring occurs where the upper part of the cup forms a wavy silhouette.Examining the grain structure can reveal the location of inhomogeneities in the workpiece and help diagnose the root cause.
Proper annealing is critical to achieving isotropy, but it is also important to understand the extent of deformation prior to annealing.As the material plastically deforms, the grains begin to deform.In the case of cold rolling, converting thickness to length, the grains will elongate in the rolling direction.As the aspect ratio of the grains changes, so do the isotropy and overall mechanical properties.In the case of heavily deformed workpieces, some orientation may be retained even after annealing.This results in anisotropy.For deep-drawn materials, it is sometimes necessary to limit the amount of deformation before final annealing to avoid wear.
orange peel.Picking up is not the only deep-drawing defect associated with die.Orange peel occurs when raw materials with too coarse particles are drawn.Each grain deforms independently and as a function of its crystal orientation.The difference in deformation between adjacent grains results in a textured appearance similar to orange peel.Texture is the granular structure revealed on the surface of the cup wall.
Just like the pixels on a TV screen, with a fine-grained structure, the difference between each grain will be less noticeable, effectively increasing the resolution.Specifying mechanical properties alone may not be sufficient to ensure a sufficiently fine grain size to prevent the orange peel effect.When the change in workpiece size is less than 10 times the grain diameter, the properties of the individual grains will drive the forming behavior.It does not deform equally over many grains, but reflects the specific size and orientation of each grain.This can be seen from the orange peel effect on the walls of the drawn cups.
For an ASTM grain size of 8, the average grain diameter is 885 µin.This means that any thickness reduction of 0.00885 inches or less can be affected by this microforming effect.
Although coarse grains can cause deep drawing problems, they are sometimes recommended for imprinting.Stamping is a deformation process in which a blank is compressed to impart a desired surface topography, such as a quarter of George Washington’s facial contours.Unlike wire drawing, stamping usually does not involve a lot of bulk material flow, but does require a lot of force, which may just deform the surface of the blank.
For this reason, minimizing surface flow stress by using a coarser grain structure can help alleviate the forces required for proper mold filling.This is especially true in the case of free-die imprinting, where dislocations on surface grains can flow freely rather than accumulating at grain boundaries.
The trends discussed here are generalizations that may not apply to specific sections.However, they did highlight the benefits of measuring and standardizing raw material particle size when designing new parts to avoid common pitfalls and optimize molding parameters.
Manufacturers of precision metal stamping machines and deep-drawing operations on metal to form their parts will work well with metallurgists on technically qualified precision re-rollers who can help them optimize materials down to the grain level.When metallurgical and engineering experts on both sides of the relationship are integrated into one team, it can have a transformative impact and produce more positive outcomes.
STAMPING Journal is the only industry journal dedicated to serving the needs of the metal stamping market.Since 1989, the publication has been covering cutting-edge technologies, industry trends, best practices and news to help stamping professionals run their business more efficiently.
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