|Fig. 1. A typical material certification sheet|
When trying to determine a material’s response to heat treatment, it is important to understand the form, prior treatments, chemical composition, grain size, hardenability and perhaps even the mechanical properties from which the parts were manufactured. The certification sheet for the material in question supplies such information. Sadly, they are seldom consulted until after a problem has occurred. It’s time to learn what these documents are, why they are so useful and how to interpret them. Let’s learn more.
Form and Prior (Mill) Treatment
Knowing the form, size and origin of the raw material can help the metallurgist or heat treater anticipate how the material will behave during manufacturing and change during heat treatment. For example, the material may be hot or cold rolled and be supplied from bar stock, tubing, wire, strip or plate. The material may be wrought, cast, forged or made from powder-metallurgy methods. The material certification sheets also tell you the source and prior mill treatment. A forging may have been normalized at the mill or may need this type of treatment before manufacturing component parts. Bar stock may be annealed to a lamellar or spheroidized structure for machining or wire partially annealed after being drawn.
Steelmaking Process and Applicable Standards
There can be subtle but meaningful differences in products made via Basic Oxygen Furnaces (BOF), primarily hot-metal-based steelmaking, and those made via Electric Arc Furnaces (EAF), primarily scrap-based practices. The material may be aluminum killed (fine grain) or silicon killed (coarse grain) and/or treated by various other elemental additions (e.g., calcium, tellurium). ASTM, AMS, AISI, SAE or other similar U.S. or international standards may be called out and should be consulted prior to heat treatment.
Different chemical elements influence the response of a material to heat treatment (Table 1). In general, the greater the amount (e.g., weight percentage) of the alloying element(s), the more pronounced the effect would be. For example, greater strength is achieved by adding carbon (C), manganese (Mn) or nickel (Ni). Corrosion resistance can be enhanced by adding chromium (Cr) or copper (Cu). Machinability is improved by adding lead (Pb), sulfur (S) or selenium (Se). High-temperature properties are retained by adding tungsten (W) or molybdenum (Mo). Most of the alloying elements, either singularly or in combination, service more than one of these purposes.
Grain size can have a significant effect on heat treatment. Steels with ASTM grain size 1-4 are considered coarse grain while 5-8 are considered fine grain (Table 2). Large (coarse) grain size is generally associated with greater hardenability but lower hardness (strength) and ductility. In heat-treated steels, the grain size after heat treatment (typically but not always martensite) is not readily measured. Instead, we measure the size of the prior austenite grains since it can be correlated to the properties of the heat-treated steels. Special etching procedures may well be needed to reveal these prior grain boundaries.
Many of the important mechanical properties of steel, including yield strength and hardness, the ductile-brittle transition temperature and susceptibility to environmental embrittlement, can be improved by refining the grain size. The improvement can often be quantified using the Hall-Petch relationship. The quantitative improvement in properties varies with d-1/2, where d is the grain size. There are special techniques to further reduce the grain size. The most common is the use of multiple quenches. This involves repeating the austenizing and quenching process several times.
Hardness and Hardenability
Material certification sheets usually provide a report on the hardness and hardenability of the material after mill processing and, when specified, supply information on the hardenability of the material (by providing Jominy and/or Ideal Diameter (DI) values) and in some cases on Carbon Equivalence (CE). Hardness is a measure of how hard or strong the material is, while hardenability may be thought of as the property that determines the depth and distribution of hardness when steel is austenitized and quenched.
Steel cleanliness is one measure of steel quality. The content of elements such as phosphorus, sulfur, total oxygen, nitrogen and hydrogen are usually, but not always, minimized. Likewise, the amount, morphology and size distribution of various species of nonmetallic inclusions should generally be minimized (Table 3). It is well known that the individual or combined effect of carbon, phosphorus, sulfur, nitrogen, hydrogen and total oxygen in steel can have a remarkable influence on steel properties, such as tensile strength, formability, toughness, weldability, cracking resistance, corrosion resistance and fatigue resistance.
When requested or specified, the common mechanical properties shown on material certification sheets include some combination of strength, ductility and/or toughness. Examples include tensile and yield strength, percent elongation, reduction in area and Charpy values. If the customer requests special testing, these results are reported as well. In other instances, the customer may request specific mechanical testing, which will also be reported on the material certification sheet. The heat treater should pay particular attention to all mechanical-property data when designing his recipes and choosing his equipment and processing parameters.
Material certification sheets are an invaluable tool for metallurgists and heat treaters and should be consulted before any heat treatment is performed on every load of parts. In this way, our recipes and cycles can be optimized to take into account the particular circumstances surrounding how the steel was specified and produced. IH