February 2021 Volume 3

By Nick Cerwin and Benjamin Ritchey

MATERIALS

Optimizing Grain Flow and Cavity Alignment of Forging Dies By Nick Cerwin and Benjamin Ritchey In our last article (FIA Magazine, November 2020) we discussed some fundamentals of grain flow and how it impacts the mechanical properties of die steel. With this article, we dive deeper into how you can work with grain flow to optimize your die design.

blunt U-notch, no-notch, etc.). For the purposes here, fracture toughness is fundamentally the concept of work (energy) in ft.-lbs. necessary to propagate the crack across a certain area. For die steel, two common methods of toughness measurement are routinely performed. A standard uniaxial tensile test provides the steel toughness for slow strain rate conditions (as experienced on a hydraulic or mechanical press) by calculating the area under the tensile stress-strain curve. A Charpy (usually V-notch) impact test provides toughness for high strain-rate conditions (as experienced on hammers or screw presses). Table 1 shows relative tensile and Charpy impact test values comparing longitudinal test measurements with long transverse (die width test direction) and short transverse (die thickness test direction) for a typical steel with average micro cleanliness (nonmetallic inclusion content). For die steel, two common methods of toughness measurement are routinely performed. A standard uniaxial tensile test provides the steel toughness for slow strain rate conditions (as experienced on a hydraulic or mechanical press) by calculating th area under the tensile stress-strain curve. A Charpy (usually V-notch) impact test provides toughness for high strain-rate conditions (as experienced on hammers or screw presses). Table 1 shows rel tive tensile and Charpy impact test values comparing longitudinal test measurements with long transverse (die width test direction) and short transverse (die thickness test direction) for a typ cal steel with av age micro cleanliness (nonmetallic inclusion content). (sharp V-notch, blunt U-notch, no-notch, etc.). For the purposes here, fracture toughness is fundamentally the concept of work (energy) in ft.-lbs. necessary to propagate the crack across a certain area.

In our last article (FIA Magazine, November 2020) we discussed some fundamentals of grain flow and how it impacts the mechanical properties of die steel. With this article, we dive deeper into how you can work with grain flow to optimize your die design. Grain flow in die steel makes the orientation of the impression relevant to die life. Each closed die impression imposes a pattern of stresses largely unique to each job and aligning the impression to take advantage of die steel grain flow characteristics will maximize die life. Although the ultimate tensile (breaking) strength of die steel does not change with stress direction, the ductility does. Fracture toughness of steel is a combination of tensile strength and ductility, so resistance to crack propagation varies in different directions. There are various methods for determining fracture toughness, and each one may yield slightly different results depending upon specific testing conditions (elastic, plastic) and notch type (sharp V-notch, Grain flow in die steel makes the orientation of the impression relevant to die life. Each closed die impression imposes a pattern of stresses largely unique to each job nd aligning the impression to take advantage of die steel grain flow characteristics will maximize die life. Although the ultimate tensile (breaking) strength of die steel does not change with stress dir ction, the ductility does. Fracture toughness of steel is a combinati n of t nsile strength and ductility, so resistance to crack propagation varies in different directions. There are various methods for determining fracture toughn ss, and each e may yield slightly different results depending upon specific testing conditions (elastic, plastic) and notch type

Table 1. Relative influence of grain flow on the mechanical properties of a typical steel with average microcleanliness Test Direction Measured Property Longitudinal (L) Long Transverse (W) Short Transverse (H) Tensile Strength 1.00 1.00 1.00 Yield Strength 1.00 1.00 1.00 Elongation 1.00 0.70 0.50 Reduction of Area 1.00 0.70 0.50 Charpy V-notch Energy 1.00 0.70 0.50

FIA MAGAZINE | FEBRUARY 2021 52 Although impact testing results are influenced by the same parameters as tensile testing, and generally follow similar trend , the ctual results are mor strongly influenced by test temperature, grain size, and microstructure, much more so than a tensile test. So, the simple relationship shown in Table 1 does not reflect the range of possible results from di fering test methods. But within the standardized laboratory procedures of a Charpy impact test, effects of grain flow are clearly evident on the “upper shelf”, or ductile portion of a Charpy V-notch (CVN) FATT curve (see FIA Magazine August 2019, Effect of Ductile-to-Brittle Transition Temperature on Die Life ). Upper plateau energy losses for a CVN under laboratory conditions experience about Although impact testing results are influenced by the same parameters as tensile testing, and generally follow similar trends, the actual results are more strongly influenced by test temperature, grain size, and microstructure, much more so than a tensile test. So, the simple relationship shown in Table 1 does not reflect the range of possible results from differing test methods. But within the standardized laboratory procedures of a Charpy impact test, effects of grain flow are clearly evident on the “upper shelf ”, or ductile portion of a Charpy V-notch (CVN) FATT curve (see FIA Magazine August 2019, Effect of Ductile-to-Brittle Transition Temperature on Die Life). Upper plateau energy losses for a CVN under laboratory conditions experience about the same energy losses as those in Table 1 for the tensile test. The lower plateau of the CVN is brittle fracture, which is minimally influenced by micro

inclusions and grain flow direction. Maintaining the die operating temperature above the FATT, together with aligning high-stress planes perpendicular to the grain flow direction is the best approach to minimize cracking. For many die impressions, the advantage of having the grain flow in one direction or another is fairly obvious. For example, piston rods, wrenches or any similar high-aspect ratio forgings usually benefit from a cross-grained flow, i.e., across the width of the die (and impression). Such impressions commonly have lengthwise features that serve as stress-risers and become high potential cracking sites. However, actual forging conditions are sometimes needed to identify die weak points that may not have been anticipated. For example, hammer dies are usually sunk with the grain flow in the front-to-back direction. But if early cracking is a problem at a shank the same energy losses as those in Table 1 for the tensile test. The lower plateau of the CVN is brittle fracture, which is minimally influenced by micro-inclusions and grain fl w direction. Maintaining the die operating temperature above the FATT, together with aligning high-stress planes perpendicular to the grain flow direction is the best approach to minimize cracking. For many die impressions, the advantage of having the grain flow in one directio or another is fairly obvious. For example, piston rods, wrenches or any similar high aspect ratio forgings usually benefit from a cross-grained flow, i.e., across the width of the die (and impression). Such impressions commonly have lengthwise features