May 2021 Volume 3

MATERIALS

formation upon quenching. The alloying elements suppress the diffusional transformation of austenite thereby permitting martensite formation at slower cooling rates. In other words, high hardenability allows the slower interior cooling rate of a thick block to develop a high percentage of the desirable microstructure. The key mechanical properties of impact strength, yield-to-tensile strength ratio, and fatigue strength are all proportionately improved with a martensitic microstructure. A martensitic matrix is also key to obtaining a good dispersion of desirable alloy carbides upon tempering. Even relatively small die blocks have larger cross-sections than most commercial steel products, which poses a challenge to achieving a good microstructure. A primary objective for alloying, therefore, is increased hardenability to provide better hardness and microstructure deeper into products with larger cross-sections, like die blocks. Table 1 shows the DI for several grades and highlights how increasing alloy content significantly increases the ability of steel to fully harden through thick cross sections. For a fuller discussion of hardenability, please see Alloying Die Steels for Strength, Toughness and Temper Resistance, FIAMagazine, August 2020, p. 37.

carbides of a critical size throughout the iron matrix inhibits crystal deformation along slip planes, strengthening the steel. Carbide distribution and size are highly dependent on thermal history, and precise control is required throughout the manufacturing process to reach the desired steel properties. The initial hardening process for carbon and alloy steel involves the formation of martensite during the quenching process. This condition is simply carbon and certain alloys that are trapped in inter-atomic spaces of the iron crystal. Until thermal energy is applied (heating), no chemical reaction occurs between carbon and iron, or the alloys. This is a highly stressed condition that produces hard, brittle steel. Upon heating, sufficient thermal energy is available to trigger the iron-carbon reaction at around 400°F. These iron carbides hinder the iron lattice from slipping and deforming, i.e., yielding. But as the steel rises to temperatures above about 500°F, the iron carbides begin to coalesce to larger sizes. This coarsening process (termed tempering ) lowers the effective strengthening capacity of iron carbide, leading to softer steel. Alloy carbides, however, begin to precipitate at significantly higher temperatures: around 1000-1100°F. This creates a higher temperature, secondary hardening process eminently suitable for die steel service. With increased time at high temperature or with further increasing temperatures, alloy carbides will also coalesce or change composition and morphology to become less effective as strengthening particles. Dwelling at such high temperatures will result in a softening effect, as with carbon steel at lower temperatures. However, due to the high temperature formation and stability of alloy carbides, die steel alloys tempered initially in the range of 1000-1100°F will resist softening at moderately high service temperatures (500-1000°F). This effect with alloy steel is termed temper resistance . Incremental increases in maximum service temperature within and just beyond this range are achieved through changes in the alloy composition to promote preferred carbide compositions. However, in all cases, achieving maximum die life is only possible by maintaining die operating temperatures within or to the low side of the tempering range. The die steel alloys are only temper resistant, not temper immune. Ameasured and regular application of coolant/lubrication should be used to control die temperatures. Additional guidance on temper resistance and controlling die temperature can be found in Controlling Maximum Die Temperature for Better Die Life, FIA Magazine, November 2019, p. 28 and Methods and Practices for Controlling Die Temperatures , FIAMagazine, February 2020, p. 40. Alloys added for carbide formation in die steel also act as ferrite stabilizers . These alloys raise the temperature at which ferrite – the stable structure of the iron matrix in alloy steel at room temperature – transforms to austenite (i.e., they increase Ac1 temperature).When this phase transformation event occurs in an uncontrolled manner during service, it erases the existing heat-treated microstructure of the die steel, and transforms it upon cooling to a new, less optimum microstructure. Typically, these effects are limited to the immediate surface layers, but the higher transformation temperature from higher alloy can be critical in preventing transformation that leads to heat-checking. A typical reaction of a forge team to a hot-running

Table 1. Key alloying elements and calculated DI of representative die steels and the common engineering steels 4140 and 4340. Metal carbides form at different stages of steel processing, from its initial solidification to final heat treatment. The metallic composition of the carbide, its morphology, distribution, and thermal stability are all factors in how carbides contribute to die steel properties. In general, these characteristics are tailored through the amount of each carbide-forming alloy present in each die steel grade, the temperatures and times used in thermomechanical processing (forging or rolling), and the heat-treatment process (annealing, austenitizing, quenching, and tempering). The most commonly employed alloys for dies used in closed-die forging are molybdenum (Mo), vanadium (V) and chromium (Cr). Referring again to Table 1, one can see the relative amounts of these elements in the different die steels. Generally speaking, increased carbide density benefits wear resistance regardless of the carbide type. This is due to the alloy carbides having inherently higher hardness relative to the surrounding iron matrix. Additionally, a fine, uniform dispersal of

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FIA MAGAZINE | MAY 2021