August 2021 Volume 3


Question: Can you explain the differences between various types of stainless steels used for forging alloys and how they are heat treated? Oxidation and corrosion are common root causes of component failures. Therefore, stainless steel forged products serve critical functions in almost every industry. Functionally, components made from these alloys are designed with corrosion resistance as the critical performance feature, but the goal of this article (and the next!) is to elaborate upon some of the finer details of these alloy systems that often get taken for granted, especially with respect to heat treatment. As this is a broad topic, the next two Heat Treating Corner editions will provide a brief overview of stainless steel classifications and describe how the various grades of stainless steel are designed to provide certain properties to the end user. Special focus will be given to critical heat treating parameters for each classification, especially how the heat treatment directly supports the final properties of interest. Chromium content is the critical defining feature to anything referred to as a “stainless” alloy. The generally accepted minimum level of chromium content needed to impart the necessary level of corrosion resistance is 11-12%. Functionally, the presence of chromium in a sufficient amount allows for the formation of an invisible chromium-rich oxide surface film on all surfaces of the material. This surface film can effectively heal itself after being exposed to oxygen environments. As will be discussed later, the heat treating process can affect the quality of this passive oxidation layer and, at the same time, improve other performance characteristics of the material. In order to understand the design theory (and therefore value) of stainless steels, it is first important to understand in basic terms how oxidation and corrosion cause failure of steel components. Without design interventions such as alloying, plating, or coatings, steels are highly susceptible to corrosion or oxidation (elevated temperatures). Components that are not designed or adequately processed to provide protection from corrosive environments will fail due to chemical alteration of surfaces that may result in these common corrosion related failure modes: • Bulk material removal (general corrosion) • Surface pitting (localized attack) • Intergranular corrosion (breakdown between steel grains) • Stress corrosion cracking (cracking that is accelerated in a corrosive environment due to tensile stress) • Erosion corrosion (corrosion activity enhanced by abrasive wear). Stainless steels, when applied and processed correctly , are engineered to resist some or all of these common failure modes without the need Heat Treat Corner By Chuck Hartwig

for secondary plating or coating operations. The most common stainless grades for forging alloys are martensitic, austenitic, duplex, and precipitation hardening. Each of these have unique concerns for product application and heat treatment parameters which will be discussed individually below. This column will discuss martensitic and austenitic grades – next issue will cover duplex and precipitation. The first grade to consider are the martensitic stainless steels. Typically, these are referred to as “400 series” stainless steels, with 410, 426, 420, and 440A/B/C being most common. Of all the stainless steel grades, these are most similar to common alloy steel grades (such as 4340) in that they have a readily attainable hardening response to heat treatment and maintain a body centered tetragonal (bct) crystal structure. Most commonly, these alloys are processed using a quench and temper heat treatment and would have bulk physical properties like alloy steels for any given hardness level but with significant enhancements to corrosion resistance. Being that these materials contain between 11.5 and 18% chromium means that they have extremely high hardenability. The carbon content varies between ~.15%(for 410/416) up to over 1.00%(440), so a wide range of usable hardnesses are available to the end user. Martensitic stainless steels are useful in various high pressure valve applications (higher toughness), firearm barrels (good strength and toughness), and in components like cutlery, medical implements, and ball bearings where higher hardnesses are required in conjunction with corrosion resistance. Typically, martensitic stainless steels can be sufficiently heat treated by heating to the proper austenitizing temperatures (around 1800°F typical) and cooling in air. The extremely high Cr content means that even slow cooling rates can achieve through-hardening on thick cross sections. Oil quenching may be advisable on heavy cross sections of the lower carbon grades such as 410. Proper heat treatments of martensitic stainless steels accomplish two critical things. First, it provides the necessary hardening/strengthening reaction by creating a final bulk microstructure of tempered martensite. Second, it provides for enhanced corrosion protection by keeping chromium atoms in the atomic matrix and thus available for formation of a continuous passive oxidation layer. Austenitic stainless steels are perhaps the most common stainless steel grade. These steels have similar Cr levels to the martensitic grades; however, nickel is added in large amounts – typically 10 to 15%. The large nickel addition increases corrosion resistance but also means that these alloys maintain an austenitic microstructure even at room temperature. The results is that these alloys have rather low strength levels (less than half ) as compared to the stainless grades. On a positive note, they have extremely high ductility and toughness and are not nearly as susceptible to cold temperature


Made with FlippingBook Ebook Creator