Make Sure Your Specified Heat Treatment is Achievable ased on many years of expe- is a function of the carbon content of rience as a commercial heat the steel being used. For example, if treater, the most common the material selected is 4140 having a Failure to consider the mistakes made by engineers carbon range of 0.37-0.44%, one B would expect the surface to be ap- statistical nature of and customers (mostly with smaller material chemical companies) are in specifying mate- proximately 99.9% martensite after rials, defining the required heat treat- quenching. Thus, the expected as- compositions and ment processes, and / or specifying quenched hardness would be 54-58 processes can lead to reasonable tolerances for the desired HRC for all the heats of 4140H that are hardness or case depth results. available. The ability to achieve this writing unrealistic heat These mistakes result generally hardness would be dependent on the treating specifications. from a failure to recognize the statis- severity of quenching, the quenchant To achieve consistent tical nature of both materials chem- used, and the adequate removal of ical composition (specifically steels) surface decarburization that might be results that meet the and processes. Often, the proper tol- present on the wrought steel product. specification after heat erances for the specified heat treat In the case of furnace heat treating, properties are either too ambiguous the maximum attainable surface treatment, the or too restrictive from a realistic hardness depends on steel carbon specification must include standpoint. There are a few basic content and the hardenability of the tolerances that are metallurgical guidelines that can, steel. The maximum section size that from a practical standpoint, help to will harden through to maximum achievable on a avoid these problems. surface hardness in oil is shown commercial basis. below Steel Selection Problems in specifying a steel Maximum Steel grade section size, in. Jon L. Dossett, P.E.* grade for a particular application generally involve selecting a steel 1045 0.250 Consultant, Chandler, Ariz. 5140 0.750 grade that cannot be hardened to the 4140 1.5 specified hardness specified and 4340 3.0 specifying a surface or core hardness range that is too restrictive. Table 1 The commercially accepted range provides a guideline for the max- for surface hardness after tempering imum surface hardness achievable is 5 HRC points or 40 BHN points as * Fellow and Life member of using induction hardening or an ap- shown in Table 2. ASM International, and member, plied surface-heating hardening Determining the expected core ASM Heat Treating Society treatment. The maximum hardness hardness range can be determined Table 1 — Carbon content vs. hardness for different martensite levels [1]
Carbon Rockwell hardness for percent martensite Carbon Rockwell hardness for percent martensite content, content, wt% 80% 90% 95% 99.9% wt% 80% 90% 95% 99.9% 0.31 42 45 47 51 0.45 49 53 55 58 0.32 43 46 48 51 0.46 50 53 56 59 0.33 43 46 48 52 0.47 50 54 56 59 0.34 44 47 49 52 0.48 51 54 57 59 0.35 44 47 49 53 0.49 51 54 57 60 0.36 44 47 49 53 0.50 52 55 58 60 0.37 45 49 51 54 0.51 52 55 58 60 0.38 46 49 51 55 0.52 53 55 58 61 0.39 46 50 52 55 0.53 53 56 59 61 0.40 47 50 52 56 0.54 53 56 59 61 0.41 47 51 53 56 0.55 54 56 59 61 0.42 48 51 54 57 0.56 54 56 60 62 0.43 48 52 54 57 0.57 54 57 60 62 0.44 49 52 55 58 0.58 55 57 60 62
HEAT TREATING PROGRESS • MARCH/APRIL 2007 23 Table 2 — Commercially acceptable print specification by using the Jominy equivalent tolerances for properly specified materials cooling rate (JEC) chart (Fig. 1) and the hardenability band for the steel Process Variable Req. tolerance range being used (Fig. 2). JEC curves for Hardening Surface hardness 5 HRC points 4140H show that using a strong oil Surface hardness 40 BHN points(a) quench results in a surface hardness Core hardness JEC “H” range for a 2 in. round would be at J4 and Case hardening Surface hardness (60 HRC) 2 HRC points the core would at J8.5. The harden- Surface hardness (HRC 50-59) 3 HRC points ability curves show that the corre- Case depth: <0.010 in. 0.004 in. sponding surface hardness would be Case depth: 0.011-0.040 in. 0.005 in. 51-59 HRC with a core hardness of Case depth: 0.025-0.050 in. 0.007 in. 46-57 HRC. The actual hardness of a Case depth: >0.050 in. 0.010 in. given heat of steel would depend Surface carbon levels 0.10% C strictly on the specific hardenability (a) Must use Brinell numbers corresponding to BID in 0.05 mm increments. of that heat. To reduce the variation in hard- Center ening response and thus narrow the
0.50 0.35 0.20 H value Quench Agitation surface and core hardness values that 1/2 in. 1/2 radius 0.20 Oil No rd. result after heat treatment, the hard- 0.35 Oil Moderate enability of the steel chosen can be 0.50 Oil Good 1.0 1.5 0.70 Surface 0.70 Oil Strong narrowed by either using an “H” steel or using a restricted harden- Center 1.0 Water No
2.0 1.5 Water Strong ability specification of an “H” steel. 0.50 0.35 0.20 2.0 Brine No The effect on hardenability of an 8620 3/4 in. 1/2 radius 5.0 %rine Strong rd. steel having a specified chemical f Ideal composition versus 8620H is shown quench 1.0 1.5 0.70 Bar diameter, in. Bar diameter, Surface in Fig. 3. The effect of changing to 8620H for carburized parts having a Center f
1.0 cross section less than 1 in. is a reduc- 0.50 0.35 0.20 tion of up to 30% in the variation in 1 in. rd. 1/2 radius core hardness after heat treatment and greater dimensional stability.
1.5 5.0 2.0 0.70 Surface Generally, you want to select the 0 4 8 12 16 20 24 28 32 most economical steel that when heat Jominy distance, 1/16 in. treated will achieve the desired prop- Fig. 1 — Jominy equivalent cooling rate by bar size and quench severity [2]. erty or properties such as surface and/or core hardness, tensile and/or Hardness limits for UNS H86200 Hardenability band SAE/AISI 8620H specific purposes yield strength, etc. To select the “J” dis- CMnSiNiCrMo proper steel, one should first check tance, 0.17/0.23 0.60/0.95 0.15/0.35 0.35/0.75 0.35/0.65 0.15/0.25 sixteenths the maximum attainable hardness of an 8620 H chart such as Table 1 using the carbon inch Max Min Diameters of rounds with same as-quenched hardness, in. Location in round Quench 14841 24 Surface Mild range that will give the required sur- 24737 1 2 3 4 3/4 radius from center water 34432 0.5 1 1.5 2 2.5 3 3.5 4 Center quench face hardness, from the 95 or 99.9 % 44127 1 2 3 4 Surface 53723 Mild oil martensite column. Following these 0.5 1 1.5 2 2.5 3 3.5 4 3/4 radius from center 63421 quench 732— 0.5 1 1.5 2 2.5 3 3.5 Center guidelines will also result in greater 830— dimensional stability and uniformity 929—65 10 28 — after heat treatment. 11 27 — 60 12 26 — 13 25 — 55 Resulfurized Steel-Hardenability 14 25 — 15 24 — 50 Problems 16 24 — 18 23 — 45 A factor that is often overlooked in 20 23 — specifying resulfurized steels for case 22 23 — 40 24 23 — hardening processes is the interrela- 26 23 — 35 28 22 — tionship and reaction of sulfur and 30 22 — 32 22 — 30 manganese that can have a signifi- Rockwell Hardness C scale Heat Treating 25 cant effect on both case and core Temperatures recommended by hardenability and microstructure. SAE 20 *Normalize 1700°F 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 All steels contain very small Austenitize 1700°F Distance from quenched end, sixteenths of an inch *For forged or rolled amounts of sulfur, but certain grades specimens only Fig. 2 — Jominy hardenability data for SAE 8620 steel [3]. of steels have much larger quantities 24 HEAT TREATING PROGRESS • MARCH/APRIL 2007 of sulfur present, which combines with some of the manganese to form 50 the compound manganese sulfide (MnS). The manganese sulfide serves as a chip breaker and lubricity agent that cause these steel grades to be more machinable than other stan- 40 dard lower sulfur steels. The formation of MnS as inclu- 8620H sions has some negative effects in- 8620 cluding: • Reduction the fatigue limit com- 30 pared with low sulfur steels and the Rockwell C hardness tendency “banding” in the steel in the as-rolled condition. • Removal of a substantial part of the manganese as manganese sulfide 20 lowers the hardenability of the steel. 1 2 3 4 5 6 7 8 9 10 11 12 The reduction of manganese avail- Jominy distance, 1/16 in. able to aid in hardenability is directly Fig. 3 — Comparison of Jominy hardenability curves for SAE 8620H and 8620 steels; chem- related to the amount of sulfur ical composition at maximum and minimum of the composition range. present. The interaction of man- This calculation can be used to de- ganese and sulfur when they com- termine the amount of manganese bine to form manganese sulfide is de- available for hardenability for some termined by the ratio of atomic common steels used for case hard- weights of Mn (56) and S (32), or ening and induction processing as 56/32; i.e., 0.0175 wt% of the man- shown Table 3. The available man- ganese combines with each 0.01 wt% ganese and other alloying elements, of sulfur present. as well as grain size, influence case
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HEAT TREATING PROGRESS • MARCH/APRIL 2007 25 Table 3 — Manganese available for hardenability after being heat treated to a speci- fication stating: “Case harden to for some common heat treated steels 0.015-0.020 in. depth. Surface RC 58 Steel grade Specified Mn, wt% Tied up as MnS, wt% Available Mn, wt% minimum.” The specification was 12L14 0.85-1.15 0.45-0.61 0.24-0.70 unclear to the heat treater, although 1117 1.00-1.30 0.14-0.23 0.77-1.16 the specification was not questioned 1018 0.60-0.90 0-0.09 0.51-0.90 until litigation ensued as a result of 1141 1.35-1.65 0.14-0.23 1.12-1.51 a part failure. 1144 1.35-1.65 0.42-0.58 0.78-1.23 1045 0.60-0.90 0-0.09 0.51-0.90 There are several technical prob- lems with the specification regarding Table 4 — Minimum effective case depth required process definition, case depth defi- nition, and the use of an improper for accurate hardness test results hardness scale to measure the hard- Effective case depth(a), in. Hardness test method ness of case depth. The discussion in- <0.010 File test or microindentation volved what each party meant or in- 0.011-0.015 Rockwell 15 N scale terpreted from the specification when 0.016-0.018 Rockwell 30 N scale the real problem was that the met- 0.019-0.021 Rockwell 45 N scale allurgical requirements were not 0.022-0.027 Rockwell A scale >0.028 Rockwell C scale clearly defined or communicated on the drawing. As a result, the bearing (a)Case hardness must be 55HRC min. manufacturer wrote heat treat spec- To reduce the variation and core hardenability. However, for ifications covering all heat treatments the steels listed, manganese is the used for bearings, including micro- in hardening response principal alloying element. structural requirements for case and thus narrow the During a carburizing or carboni- hardened parts. triding treatment, control of the fur- surface and core hardness nace parameters of time, tempera- Setting Reasonable Tolerances: values that result after ture, and atmosphere carbon Attainable Hardness Values establish the carbon penetration pro- When specifying the hardness heat treatment, the file for a given steel chemistry. How- range or tolerance for either expected hardenability of the steel ever, the effective case depth (to 50 surface hardness values or core hard- HRC) depends on case hardenability ness, it is necessary to recognize the chosen can be narrowed and the core hardness achieved de- full range of hardenability of the by either using an “H” pends on hardenability. Thus, for the specified steel at the point of interest same total case carbon profile, the on, or in, the part. The expected vari- steel or using a restricted higher the case hardenability, the ation in surface hardness after induc- hardenability specification higher the effective case. For this tion or flame hardening can be deter- reason, effective case depth varia- mined by checking the high and low of an “H” steel. tions for resulfurized materials are values of the carbon for the specific significantly greater than those for grade of steel versus the hardness non-resulfurized steels. values shown in the 99.9% marten- The final negative effect of case site column in Table 1. SAE 1045 steel hardening resulfurized steels is the with a carbon range of 0.43-0.50% can large variation in case microstructure be hardened to 57-60 HRC as fully (principally with regard to retained quenched to martensite, but SAE austenite) when a constant heat 4140 steel with a carbon range of 0.38- treating process is used. The problem 0.43% can only be hardened to 55-57 can be attributed to the variation in HRC. manganese levels due to varying For through-hardening heat amounts of sulfur present. treatments, the curves in Fig. 1 can be used with the Jominy harden- Terminology ability data for the specific steel When specifying the required grade to predict the actual variation case-hardening heat treatment, it is in surface or core hardness for a important to define the process to given steel and quenching situa- be used, to define the case depth, tion. To predict the expected varia- and proper hardness testing scale to tion in core hardness at the center be used. This is illustrated in a case of 0.5 in. section of 8620H (Fig. 1), study where hardened light-duty the center of the section when bearings made of 1117 steel failed quenched in oil with good agitation 26 HEAT TREATING PROGRESS • MARCH/APRIL 2007 (H = .35) will yield hardnesses showing acceptable microstructures Specify the required equivalent to about the J3 position. that can be used by both parties From the Jominy hardenability data when doing the evaluation. heat treat process shown in Fig. 2, the hardness values results, NOT how to at the J3 position are 32-44 HRC, or Practical Considerations a 12 RC point spread. This range of If consistent results are to be do the process. core hardness values would be an achieved that meet the specification appropriate print specification after heat treatment, the specification given this specific situation. must include tolerances that are achievable on a commercial basis. Part Geometry Effects Commercially available tolerances Both the total case depth and ef- for hardness, case depth, and case fective case depth can vary on a part Continued that is uniformly heat treated due to section size, steel hardenability, and part geometry. Therefore, it is a very good practice to identify the critical area or areas where case depth or core hardness values are to be deter- mined. This identification is not so critical on gears where it is generally understood that the tests are to be at the pitch line or root diameter. For other parts, such as tubes, shafts, and parts having convex or concave spherical sections, it is very impor- tant to define the testing location. The effective case depth on a part having both a flat surface and con- cave surface can vary as much as 30% on a properly carburized part.
Hardness Testing When specifying the surface hard- ness requirements for case hardened parts, care must be taken to specify the proper hardness scale to be used. This is to ensure that the case will support the test and accurate hard- ness values are obtained. The rela- tionship between the hardness test method that can be used and the re- quired minimum specified effective case depth is shown in Table 4.
Microstructure Often, the required microstructure for a part is specified on the blue- print. It is important to recognize that the microstructure may vary signif- icantly in various areas of a part that has been properly heat treated. Therefore, it may be important to de- fine where and how the microstruc- ture is to be checked. When mi- crostructures are part of the print specification and, thus, must be eval- uated by both the supplier and cus- tomer, there is a definite need for a visual photomicrograph standard HEAT TREATING PROGRESS • MARCH/APRIL 2007 27 Table 5 — Ratio of effective- carbon levels are shown in Table 2. • Specify hardness using the to-total case depths for Other factors that should be consid- proper hardness scales with accepted ered include: evaluation techniques that are based carburize/carbonitride cycles • The required hardness scale that on standards; i.e., ASTM, SAE, or for different oil-quenched should be used to obtain accurate company specifications. 0.5 in. steel sections hardness test results (Table 4). • Specify and use “H” steels when- • The expected ratio of effective- ever possible. Specify restricted ”H” Effective case / to-total case relationships for some band hardness values for the steel if Steel grade total case depth common carburized and carboni- hardness values need to be more re- 1215 0.25-0.35 trided steels after oil quenching strictive. 12L14 0.30-0.45 1117 0.40-0.55 (Table 5). • Consider writing heat treating 1026 0.30-0.45 • The effect of lower effective-to- specifications that provide more de- 8620 0.65-0.80 total case ratios on processing time tail and definition to the required and, thus, the economics of pro- process results and how they are to Table 6 — Relative furnace cessing costs (Table 6). be measured. times for carburize / • Specify achievable heat treating carbonitride cycles for 0.030 Conclusion results recognizing the statistical The following guidelines should nature of materials and in. effective case depths in be considered to adequately and processes. 0.5 in. sections properly specify the required heat References Steel grade Furnace time, h treatment: • Specify the required heat treat 1. J I Case/IH Specification A-D, Part III, p 8, 1974. 1215 13 process results, NOT how to do the 12L14 9 2. Practical Data for Metallurgists, 1117 5 process. The exception is when you Timken Company, p53, 1991. 8620 3 know certain processing pitfalls that 3. SAE Handbook, Vol. 1, Materials, p 1.72, you wish to avoid. 1989. • Define the critical areas on the For more information: Jon Dossett is part and where the process verifica- a consultant, tel: 480-323-8185; e-mail: tion tests are to be made. [email protected].
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