hen trying to determine a materials response to heat treatment, it is important to understand the form, prior treatments, chemical composi-tion, grain size, hardenability and perhaps even

the mechanical properties from which the parts were manufac-tured. The certi cation sheet for the mate-rial in question supplies such information. Sadly, they are seldom consulted until after a problem has occurred. Its time to learn what these documents are, why they are so useful and how to interpret them. Lets learn more.

Form and Prior (Mill) TreatmentKnowing 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 certi cation 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 be-fore manufacturing component parts. Bar stock may be annealed to a lamellar or spheroidized structure for machining or wire par-tially annealed after being drawn.

Steelmaking Process and Applicable StandardsThere can be subtle but meaningful differences in products made via Basic Oxygen Furnaces (BOF), primarily hot-metal-based steel-making, and those made via Electric Arc Furnaces (EAF), primar-ily scrap-based practices. The material may be aluminum killed ( ne grain) or silicon killed (coarse grain) and/or treated by vari-ous 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.

ChemistryDifferent chemical elements in uence the response of a material to heat treatment (Table 1 - online only). 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 add-ing lead (Pb), sulfur (S) or selenium (Se). High-temperature prop-erties 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 SizeGrain size can have a signi cant effect on heat treatment. Steels with ASTM grain size 1-4 are considered coarse grain while 5-8 are considered ne 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 read-ily measured. Instead, we measure the size of the prior austenite grains since it can be correlated to the properties of the heat-treat-ed steels. Special etching procedures may well be needed to reveal these prior grain boundaries.

Material Certi cation Sheets:What They Are and How They Help

Daniel H. Herring | 630-834-3017 | heattreatdoctor@industrialheating.com

The Heat Treat Doctor

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18 December 2011 - IndustrialHeating.com

Fig. 1. A typical material certi cation sheet[1]

Many of the important mechanical properties of steel, includ-ing yield strength and hardness, the ductile-brittle transition tem-perature and susceptibility to environmental embrittlement, can be improved by re ning the grain size. The improvement can often be quanti ed 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 HardenabilityMaterial certi cation sheets usually provide a report on the hard-ness and hardenability of the material after mill processing and, when speci ed, 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.

CleanlinessSteel 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 car-bon, phosphorus, sulfur, nitrogen, hydrogen and total oxygen in steel can have a remarkable in uence on steel properties, such as tensile strength, formability, toughness, weldability, cracking re-sistance, corrosion resistance and fatigue resistance.

Mechanical PropertiesWhen requested or speci ed, the common mechanical proper-ties shown on material certi cation sheets include some combi-nation 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 speci c mechanical testing, which will also be reported on the material certi cation sheet. The heat treater should pay par-ticular attention to all mechanical-property data when designing his recipes and choosing his equipment and processing parameters.

In ConclusionMaterial certi cation sheets are an invaluable tool for metallur-gists 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 par-ticular circumstances surrounding how the steel was speci ed and produced. IH

References1. The Timken Company (www.timken.com)2. Metallographers Guide: Irons and Steels, Chapter 1: Introduction to

Steels and Cast Irons, ASM International, 2002.3. Morris, Jr., J.W., The In uence of Grain Size on the Mechanical Properties

of Steel, Lawrence Berkeley National Laboratory, 2001.4. Republic Alloy Steels, Republic Steel, 1961, pp. 247 249.5. Mr. Craig Darragh, AgFox, LLC, private correspondence.6. Clean Steel, Part One, www.keytometals.com7. www.about.com

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20 December 2011 - IndustrialHeating.com

Table 2. Austenite grains in steel (McQuaid Ehn)[3]

ASTM grain size Number of grains per in2 at 100X Diameter (in microns)

1 ) 1.5 2542 1.5-3 180

3 3-6 127

4 6-12 89.8

5 12-24 63.5

6 24-48 44.9

7 48-96 31.8

8 ) 96 22.5

Table 3. In uence of typical impurities on mechanical properties[4]

Element Form Mechanical properties in uenced

Sulfur, oxygen

Sul de and oxide inclusions

Ductility, Charpy impact value, anisotropy Formability (elongation, reduction of area and bendability) Cold forgeability, drawability Low-temperature toughness Fatigue strength

Carbon, nitrogen

Solid solution Solid solubility (enhanced), hardenability

Settled dislocation

Strain aging (enhanced), ductility and toughness (lowered)

Pearlite and cementite

Dispersion (enhanced), ductility and toughness (lowered)

Carbide and nitride precipitates

Precipitation, grain re ning (enhanced), toughness (enhanced) Embrittlement by intergranular precipitation

Phosphorous Solid solution

Solid solubility (enhanced), hardenability (enhanced)

Temper brittleness

Separation, secondary work embrittlement

Use this Mobile Tag to link to the full column containing Table 1.