|PMPA's Designer's Guide||Return to Table of Contents|
When specifying heat treating or annealing on a drawing, it is preferable to note the end condition desired rather than the process. In this way both the heat treater and the part user can check to the same standard. Spelling out the complete process will not guarantee the required end condition.
See Drawing #34 for examples of heat treatment call outs.
Use the following information as a guide to check your drawing. These tips will help assure proper heat treating and annealing.
On low carbon steel parts (1213, 12L14, 1117, 8620, etc.) specify case depth and hardness that is required.
Specified case depth should have a tolerance of .005 min. On ground parts case depth is assumed to be after grinding, but adding "after grind" will eliminate any misunderstanding.
Case depth should be stated either as total case depth, or effective case depth. Effective case depth is that portion of case that has a hardness equivalent to Rockwell "C" 50, as checked on a Tukon microhardness tester. In most cases, depth of effective case is approximately 65% of total case depth. Grade and hardenability of material, as well as total case depth, can affect this percentage. If neither total or effective case is specified, it is Industry practice to assume that total case is intended.
Generally, hardness should be specified as Rock "C'' XX-XX with a minimum spread of 5 points or a minimum hardness only (Rock "C" XX min.). However, if total case depth is less than .030", an accurate reading cannot be obtained on "C" scale because a 150 KG load will drive the penetrator through the case and into core material, giving a false reading. Following are Rockwell scales recommended for use with minimum total case depths that will give an accurate hardness reading.
Total Case Depth Rockwell Scale .030 Min. "C" Scale .024 Min. "A" Scale .021 Min. 45N Scale .018 Min. 30N Scale .015 Min. 15N Scale
Under .015 case is often specified as "file hard." This is not as accurate a method as Rockwell testing, but is usually an adequate hardness control for thin case parts.
Hardness readings taken on the O.D. of a small diameter part can be several points lower than actual hardness because of curvature of the part. A standard chart is available which shows a correction factor.
Medium to high carbon steel parts (1045, 1137, 1141 1144, 4140, 8640, etc.) have enough carbon content to harden without addition of carbon to the surface (case hardening). A grade of steel should be selected that will produce physical properties needed and quench out at a hardness above part requirement. Hardness is usually specified on Rockwell "C" scale with a minimum spread of 5 points.
When a part requires annealing, be specific as to condition and hardness required, rather than simply specifying "anneal." Hardness for annealed parts is usually checked on Rockwell "B" scale. If only one area of a hardened part is to be annealed, indicate the area and maximum hardness, with a transition zone between soft and hard areas.
On parts that are to be induction hardened, show minimum area that is to be hardened, minimum and maximum depth and any area that must remain soft, with a transition zone between soft and hard areas. The method of checking hardness should be the same as on case hardened parts.
Note: If heat treat discoloration or surface oxides are objectionable, or if decarburization is not allowed, this should be noted on the print.