November 2020 Volume 2

MATERIALS

The Fundamentals of Grain Flow in Die Steel By Nick Cerwin and Benjamin Ritchey

As a forger, you are no doubt familiar with the concept of grain flow in the products you forge. Are you aware that your forging dies also have a grain flow? Grain flow in die steel is relevant to the forging process because the fracture characteristics of a die block differ with respect to the grain direction. This can have serious consequences for die performance. So, what then is grain flow in a die? Can it be aligned in a way that avoids overlaying known or anticipated crack-prone areas of a die impression with the weakest direction of a die block? The beginning of grain flow starts with ingot solidification when long, finger-like grains develop as the liquid metal cools and transforms to the fundamental crystalline structure of iron. Initially, many small, individual grains are nucleated as the liquid steel is quickly chilled upon contact with the mold. However, this relatively thin ingot skin transitions to grain formation governed by a greatly reduced cooling rate as solidification progresses deeper into the ingot. The decelerated cooling rate favors the development of elongated, continuous, finger-like, radially aligned grains called dendrites. The dendrites usually reach mid-radius of an ingot where falling temperatures once again favor the development of smaller, mostly equiaxed grains in the core region. A die block in this cast condition would offer long, unimpeded crack paths once a crack aligns with the dendritic direction. Fracture toughness in this case would be very low, and unacceptable for die service. The unfavorable characteristics of as-cast grain structure is the reason why ingots destined for die service undergo open-die forging. The thermomechanical working of the ingot in the forging process breaks the large, as-cast grains into smaller, equiaxed grains. Subsequent heat treatments of die blocks further refine the grain size to small and mostly equiaxed grains, which are desirable conditions that minimize any impact of the original cast grain directionality. Concurrent with grain development during solidification, micro sized nonmetallic impurities agglomerate to form larger particles that establish the base micro-cleanliness of the steel. These particles, which are predominantly oxides of aluminum and silicon as well as manganese sulfides, are mostly globular in the as-cast ingot. During the forging of the ingot, these nonmetallic inclusions are also deformed and elongated. Whereas the dendritic grains are changed through the forging and recrystallization processes to end as equiaxed grains in the finished product, nonmetallic inclusions begin as largely globular shapes and end as elongated “stringers” that can be rated according to their frequency and overall length. It is this presence of elongated nonmetallic inclusions in steel that creates the grain flow character of wrought steel, not the actual grains!

Another contributing factor in developing grain flow characteristics is alloy segregation. For die steel, alloy contents are higher than most popular engineering steels, and thus tend to experience a higher degree of alloy segregation. During ingot solidification, the developing dendritic grains attempt to expel any atoms that do not exactlyfit into the growing crystalline lattice.This includes unwanted impurities as well as metallurgically beneficial atoms such as carbon, manganese and chromium. The result is a slight enrichment of alloy and impurities in the inter-dendritic spaces, and a slight depletion of alloys and impurities within the dendrites themselves. Forging the ingot works mainly to orient such segregation to the product length direction. Subsequent thermal processing promotes some mitigation through diffusion along concentration gradients, but varying degrees of microsegregation usually remain and may manifest in etched microstructures as “banding”. A crack propagating in a direction parallel to banding will follow the path of least resistance, advancing along the negatively segregated plane which has reduced strength. In contrast, a crack oriented across the banding will encounter alternating strength levels and require an overall increase in energy to propagate. Therefore, a noticeable grain effect to fracture toughness arises from the presence of microsegregation. Because forging dies experience tri-axial stress, the ideal die steel would have a complete absence of any grain flow characteristics, exhibiting equal fracture toughness in all directions. Die steel manufacturers work toward this goal by employing melting and forging procedures that significantly reduce grain flow disparities in forging dies. • Vacuum degassing: Degassing liquid steel under vacuum removes most of the oxygen, thereby reducing the presence of oxide inclusions that create grain flow effects when elongated. A secondary process occurring under an extended vacuum cycle is a significant reduction in sulfur content. Sulfur is present in steel as MnS, a highly ductile compound at forging temperatures that forms stringer-like inclusions that contribute strongly to grain flow effects. • Sulfide morphology control through calcium injection: By modifying the sulfide compound to a CaMnS-type, the compound is strengthened, thus reducing the amount it elongates during forging. With a more globular shape retained for remaining sulfides, their contribution to a grain flow effect is reduced. • Upset forging of the ingot: By standing a heated ingot upright and compressing it to half-height, a forge reduction ratio of 2:1 is affected. Turning the ingot to then

FIA MAGAZINE | NOVEMBER 2020 12