February 2021 Volume 3

MATERIALS

radius and not experienced on the impression side, then a change to cross-grain flow may be recommended to boost crack resistance on the shank side. The die user has the option of specifying the grain flow direction for most block sizes, whether individually forged or delivered from readily available bar stock. Specifying the die block dimensions is an implicit statement of grain flow direction. By convention, the last dimension specified is generally used to denote the grain flow direction. For example, specifying a 10 x 14 x 18-inch die block is understood to be requesting the grain flow in the 18-inch direction. A cross grain request should be specified as a 10 x 18 x 14-inch block. In any case, it may still be helpful to explicitly specify the desired grain flow direction for an ordered die block. Orienting the grain flow vertically creates the weakest fracture conditions and is generally to be avoided. However, it seems to work successfully on axisymmetric press inserts. Round inserts are mostly supplied from round bar stock. The lengthwise grain flow in the bar then becomes the vertical direction of the die blank as the length is cut to provide the ordered blank thickness. Depending upon experience with this popular approach, options are nevertheless available for changing the grain flow direction in round blanks. Where a cross-grained condition is desired, a rectangular bar may be cut such that the forged thickness reflects the ordered blank thickness, and the forged width and cut length reflect the ordered diameter of the blank. Then standing the square piece on edge and rounding the corners to the blank diameter leaves the grain flow along a particular diameter. Carefully marking this direction for the customer provides the option of orienting the grain flow in a direction most appropriate for their forging. Alternatively, a round bar may be forged a little under the ordered blank diameter, cutting to a length a little longer than the ordered blank thickness, then upsetting to the ordered thickness. This imparts a good degree of grain flow in a radial direction which may be a good option for any axisymmetric forging requiring added fracture toughness. An alternate approach to grain flow considerations for dies is through changes to melting techniques. It is interesting to note that advances in cutting tools have allowed a trend to lower sulfur levels in die steel without much consequence to tool life for the die sinker (lower sulfur typically decreases the machinability). Since sulfur content is largely responsible for grain flow effects, lowering the sulfur content lowers the disparity between longitudinal and transverse test results, thus narrowing the differences indicated in Table 1. While effective grain flow alignment is still relevant, lower sulfur offers improved overall toughness for tri-axial stresses.

Another option for reducing grain flow effects and boosting overall fracture toughness is to use remelted die steel. Improvements to micro-cleanliness and grain structure can be realized with either Vacuum Arc Remelted (VAR) or Electro-Slag Remelted (ESR) die steels. For especially challenging dies with high cracking potential, the added cost of remelted die steel may be cost-effective due to the inherently higher fracture toughness that results from ultra-fine grain size, and a transverse toughness that would be close to the longitudinal toughness due to lowered inclusion content and greater microstructural uniformity. ■

Figure 1. This illustration demonstrates the recommended “cross-grained” die block orientation for a high aspect ratio impression. The blue arrow indicates the direction of the grain flow in the die block.

Nick Cerwin is the retired Director of Technical Services at Finkl Steel. Benjamin Ritchey is the Technical Director for Finkl Steel and can be reached at BRitchey@Finkl.com. Nick Cerwin Benjamin Ritchey

FIA MAGAZINE | FEBRUARY 2021 53