Make Sure Your Specified Heat Treatment is Achievable Based on many years of experience

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1 Failure to consider the statistical nature of material chemical compositions and processes can lead to writing unrealistic heat treating specifications. To achieve consistent results that meet the specification after heat treatment, the specification must include tolerances that are achievable on a commercial basis. Jon L. Dossett, P.E.* Consultant, Chandler, Ariz. * Fellow and Life member of ASM International, and member, ASM Heat Treating Society Carbon content, Rockwell hardness for percent martensite wt% 80% 90% 95% 99.9% Make Sure Your Specified Heat Treatment is Achievable Based on many years of experience as a commercial heat treater, the most common mistakes made by engineers and customers (mostly with smaller companies) are in specifying materials, defining the required heat treatment processes, and / or specifying reasonable tolerances for the desired hardness or case depth results. These mistakes result generally from a failure to recognize the statistical nature of both materials chemical composition (specifically steels) and processes. Often, the proper tolerances for the specified heat treat properties are either too ambiguous or too restrictive from a realistic standpoint. There are a few basic metallurgical guidelines that can, from a practical standpoint, help to avoid these problems. Steel Selection Problems in specifying a steel grade for a particular application generally involve selecting a steel grade that cannot be hardened to the specified hardness specified and specifying a surface or core hardness range that is too restrictive. Table 1 provides a guideline for the maximum surface hardness achievable using induction hardening or an applied surface-heating hardening treatment. The maximum hardness Table 1 Carbon content vs. hardness for different martensite levels [1] is a function of the carbon content of the steel being used. For example, if the material selected is 4140 having a carbon range of %, one would expect the surface to be approximately 99.9% martensite after quenching. Thus, the expected asquenched hardness would be HRC for all the heats of 4140H that are available. The ability to achieve this hardness would be dependent on the severity of quenching, the quenchant used, and the adequate removal of surface decarburization that might be present on the wrought steel product. In the case of furnace heat treating, the maximum attainable surface hardness depends on steel carbon content and the hardenability of the steel. The maximum section size that will harden through to maximum surface hardness in oil is shown below Maximum Steel grade section size, in The commercially accepted range for surface hardness after tempering is 5 HRC points or 40 BHN points as shown in Table 2. Determining the expected core hardness range can be determined Carbon content, Rockwell hardness for percent martensite wt% 80% 90% 95% 99.9% HEAT TREATING PROGRESS MARCH/APRIL

2 Table 2 Commercially acceptable print specification tolerances for properly specified materials Process Variable Req. tolerance range Hardening hardness 5 HRC points hardness 40 BHN points(a) Core hardness JEC H range Case hardening hardness (60 HRC) 2 HRC points hardness (HRC 50-59) 3 HRC points Case depth: <0.010 in in. Case depth: in in. Case depth: in in. Case depth: >0.050 in in. carbon levels 0.10% C (a) Must use Brinell numbers corresponding to BID in 0.05 mm increments. Bar diameter, in. 1/2 in. rd. 3/4 in. rd. 1 in. rd Jominy distance, 1/16 in. Fig. 1 Jominy equivalent cooling rate by bar size and quench severity [2]. Hardness limits for specific purposes J distance, sixteenths of an 8620 H inch Max Min Heat Treating Temperatures recommended by SAE *Normalize 1700 F Austenitize 1700 F *For forged or rolled specimens only UNS H86200 Hardenability band SAE/AISI 8620H C Mn Si Ni Cr Mo 0.17/ / / / / /0.25 Rockwell Hardness C scale /2 radius /2 radius 1/2 radius Diameters of rounds with same as-quenched hardness, in H value Quench Agitation 0.20 Oil No 0.35 Oil Moderate 0.50 Oil Good 0.70 Oil Strong 1.0 Water No 1.5 Water Strong 2.0 Brine No 5.0 rine Strong Ideal quench Location in round 3/4 radius from center /4 radius from center Quench Mild water quench Mild oil quench Distance from quenched end, sixteenths of an inch Fig. 2 Jominy hardenability data for SAE 8620 steel [3]. by using the Jominy equivalent cooling rate (JEC) chart (Fig. 1) and the hardenability band for the steel being used (Fig. 2). JEC curves for 4140H show that using a strong oil quench results in a surface hardness for a 2 in. round would be at J4 and the core would at J8.5. The hardenability curves show that the corresponding surface hardness would be HRC with a core hardness of HRC. The actual hardness of a given heat of steel would depend strictly on the specific hardenability of that heat. To reduce the variation in hardening response and thus narrow the surface and core hardness values that result after heat treatment, the hardenability of the steel chosen can be narrowed by either using an H steel or using a restricted hardenability specification of an H steel. The effect on hardenability of an 8620 steel having a specified chemical composition versus 8620H is shown in Fig. 3. The effect of changing to 8620H for carburized parts having a cross section less than 1 in. is a reduction of up to 30% in the variation in core hardness after heat treatment and greater dimensional stability. Generally, you want to select the most economical steel that when heat treated will achieve the desired property or properties such as surface and/or core hardness, tensile and/or yield strength, etc. To select the proper steel, one should first check the maximum attainable hardness chart such as Table 1 using the carbon range that will give the required surface hardness, from the 95 or 99.9 % martensite column. Following these guidelines will also result in greater dimensional stability and uniformity after heat treatment. Resulfurized Steel-Hardenability Problems A factor that is often overlooked in specifying resulfurized steels for case hardening processes is the interrelationship and reaction of sulfur and manganese that can have a significant effect on both case and core hardenability and microstructure. All steels contain very small amounts of sulfur, but certain grades of steels have much larger quantities 24 HEAT TREATING PROGRESS MARCH/APRIL 2007

3 of sulfur present, which combines with some of the manganese to form 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 standard lower sulfur steels. The formation of MnS as inclusions has some negative effects including: Reduction the fatigue limit compared with low sulfur steels and the tendency banding in the steel in the as-rolled condition. Removal of a substantial part of the manganese as manganese sulfide lowers the hardenability of the steel. The reduction of manganese available to aid in hardenability is directly related to the amount of sulfur present. The interaction of manganese and sulfur when they combine to form manganese sulfide is determined by the ratio of atomic weights of Mn (56) and S (32), or 56/32; i.e., wt% of the manganese combines with each 0.01 wt% of sulfur present. Rockwell C hardness H Jominy distance, 1/16 in. Fig. 3 Comparison of Jominy hardenability curves for SAE 8620H and 8620 steels; chemical composition at maximum and minimum of the composition range. This calculation can be used to determine the amount of manganese available for hardenability for some common steels used for case hardening and induction processing as shown Table 3. The available manganese and other alloying elements, as well as grain size, influence case Pillar Induction Company Innovative and reliable induction heat treating systems for over four decades. Corporate Office: Gateway Rd. Brookfield, WI Toll-Free: Fax: sales@pillar.com ISO 9001: 2000 REGISTERED HEAT TREATING PROGRESS MARCH/APRIL

4 Table 3 Manganese available for hardenability for some common heat treated steels Steel grade Specified Mn, wt% Tied up as MnS, wt% Available Mn, wt% 12L Table 4 Minimum effective case depth required for accurate hardness test results Effective case depth(a), in. Hardness test method <0.010 File test or microindentation Rockwell 15 N scale Rockwell 30 N scale Rockwell 45 N scale Rockwell A scale >0.028 Rockwell C scale (a)case hardness must be 55HRC min. To reduce the variation in hardening response and thus narrow the surface and core hardness values that result after heat treatment, the hardenability of the steel chosen can be narrowed by either using an H steel or using a restricted hardenability specification of an H steel. and core hardenability. However, for the steels listed, manganese is the principal alloying element. During a carburizing or carbonitriding treatment, control of the furnace parameters of time, temperature, and atmosphere carbon establish the carbon penetration profile for a given steel chemistry. However, the effective case depth (to 50 HRC) depends on case hardenability and the core hardness achieved depends on hardenability. Thus, for the same total case carbon profile, the higher the case hardenability, the higher the effective case. For this reason, effective case depth variations for resulfurized materials are significantly greater than those for non-resulfurized steels. The final negative effect of case hardening resulfurized steels is the large variation in case microstructure (principally with regard to retained austenite) when a constant heat treating process is used. The problem can be attributed to the variation in manganese levels due to varying amounts of sulfur present. Terminology When specifying the required case-hardening heat treatment, it is important to define the process to be used, to define the case depth, and proper hardness testing scale to be used. This is illustrated in a case study where hardened light-duty bearings made of 1117 steel failed after being heat treated to a specification stating: Case harden to in. depth. RC 58 minimum. The specification was unclear to the heat treater, although the specification was not questioned until litigation ensued as a result of a part failure. There are several technical problems with the specification regarding process definition, case depth definition, and the use of an improper hardness scale to measure the hardness of case depth. The discussion involved what each party meant or interpreted from the specification when the real problem was that the metallurgical requirements were not clearly defined or communicated on the drawing. As a result, the bearing manufacturer wrote heat treat specifications covering all heat treatments used for bearings, including microstructural requirements for case hardened parts. Setting Reasonable Tolerances: Attainable Hardness Values When specifying the hardness range or tolerance for either expected surface hardness values or core hardness, it is necessary to recognize the full range of hardenability of the specified steel at the point of interest on, or in, the part. The expected variation in surface hardness after induction or flame hardening can be determined by checking the high and low values of the carbon for the specific grade of steel versus the hardness values shown in the 99.9% martensite column in Table 1. SAE 1045 steel with a carbon range of % can be hardened to HRC as fully quenched to martensite, but SAE 4140 steel with a carbon range of % can only be hardened to HRC. For through-hardening heat treatments, the curves in Fig. 1 can be used with the Jominy hardenability data for the specific steel grade to predict the actual variation in surface or core hardness for a given steel and quenching situation. To predict the expected variation in core hardness at the center of 0.5 in. section of 8620H (Fig. 1), the center of the section when quenched in oil with good agitation 26 HEAT TREATING PROGRESS MARCH/APRIL 2007

5 (H =.35) will yield hardnesses equivalent to about the J3 position. From the Jominy hardenability data shown in Fig. 2, the hardness values at the J3 position are HRC, or a 12 RC point spread. This range of core hardness values would be an appropriate print specification given this specific situation. Part Geometry Effects Both the total case depth and effective case depth can vary on a part 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 determined. 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 important to define the testing location. The effective case depth on a part having both a flat surface and concave surface can vary as much as 30% on a properly carburized part. Hardness Testing When specifying the surface hardness 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 hardness values are obtained. The relationship between the hardness test method that can be used and the required minimum specified effective case depth is shown in Table 4. showing acceptable microstructures that can be used by both parties when doing the evaluation. Practical Considerations If consistent results are to be achieved that meet the specification after heat treatment, the specification must include tolerances that are achievable on a commercial basis. Commercially available tolerances for hardness, case depth, and case Continued Specify the required heat treat process results, NOT how to do the process. Microstructure Often, the required microstructure for a part is specified on the blueprint. It is important to recognize that the microstructure may vary significantly in various areas of a part that has been properly heat treated. Therefore, it may be important to define where and how the microstructure is to be checked. When microstructures are part of the print specification and, thus, must be evaluated by both the supplier and customer, there is a definite need for a visual photomicrograph standard HEAT TREATING PROGRESS MARCH/APRIL

6 Table 5 Ratio of effectiveto-total case depths for carburize/carbonitride cycles for different oil-quenched 0.5 in. steel sections Effective case / Steel grade total case depth L Table 6 Relative furnace times for carburize / carbonitride cycles for in. effective case depths in 0.5 in. sections Steel grade Furnace time, h L carbon levels are shown in Table 2. Other factors that should be considered include: The required hardness scale that should be used to obtain accurate hardness test results (Table 4). The expected ratio of effectiveto-total case relationships for some common carburized and carbonitrided steels after oil quenching (Table 5). The effect of lower effective-tototal case ratios on processing time and, thus, the economics of processing costs (Table 6). Conclusion The following guidelines should be considered to adequately and properly specify the required heat treatment: Specify the required heat treat process results, NOT how to do the process. The exception is when you know certain processing pitfalls that you wish to avoid. Define the critical areas on the part and where the process verification tests are to be made. Specify hardness using the proper hardness scales with accepted evaluation techniques that are based on standards; i.e., ASTM, SAE, or company specifications. Specify and use H steels whenever possible. Specify restricted H band hardness values for the steel if hardness values need to be more restrictive. Consider writing heat treating specifications that provide more detail and definition to the required process results and how they are to be measured. Specify achievable heat treating results recognizing the statistical nature of materials and processes. References 1. J I Case/IH Specification A-D, Part III, p 8, Practical Data for Metallurgists, Timken Company, p53, SAE Handbook, Vol. 1, Materials, p 1.72, For more information: Jon Dossett is a consultant, tel: ; jdossett@cox.net. Quality Tool Steel requires Quality Heat Treatment Böhler-Uddeholm Thermo-Tech delivers state-ofthe-art heat treatment and renowned quality and customer service. Contact Böhler-Uddeholm Thermo-Tech today...because your tooling is only as good as its heat treatment. Here s Why Our Customers Choose Thermo-Tech High Pressure Vacuum Hardening Atmosphere and Vacuum Tempering Cryotherm Cryogenic Treatments Thermonite Ferritic Nitro-Carburizing Tool Steel Welding Full Service Laboratory Testing PLEASE CALL US AT OR VISIT US AT ISO / TS : 2002 CERTIFIED 30 HEAT TREATING PROGRESS MARCH/APRIL 2007