1、13FTM22 AGMA Technical Paper Heat Treatment of Large Components By G.L. Reese, Hrterei REESE Bochum 2 13FTM22 Heat Treatment of Large Components Gerhard L. Reese, Hrterei REESE Bochum The statements and opinions contained herein are those of the author and should not be construed as an official acti
2、on or opinion of the American Gear Manufacturers Association. Abstract Large gear components can be offered in many applications such as in marine, wind power, steel rolling mills, power plants, transportation, railroad, aircraft, cement crushers, mining and oil industry applications. There are thre
3、e important surface hardening methods used to improve and expand the technical use of gear components. Design and material engineers must decide which hardening method to use. Case hardening is normally the first choice because of the highest load capacity. But, case hardening also poses challenges
4、that must be acknowledged. Therefore, it is good to know that there are three options for very large components. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 ISBN: 978-1-61481-079-7 3 13FTM22 Heat Treatment of Larg
5、e Components Gerhard L. Reese, Hrterei REESE Bochum Introduction Large gear components can be offered in many applications such as in marine, wind power, steel rolling mills, power plants, transportation, railroad, aircraft, cement crushers, mining and oil industry applications. There are three impo
6、rtant surface hardening methods, as seen in Figure 1, used to improve and expand the technical use of gear components. Design and material engineers must decide which hardening method to use. Case hardening is normally the first choice because of the highest load capacity. But, case hardening also p
7、oses challenges that must be acknowledged. Therefore, it is good to know that there are three options for very large components. So first, lets compare these methods. Three methods compared Case hardening is normally carried out at temperatures between 880C to 980C for carburizing, and 780C - 860C f
8、or hardening. The standard or normal procedure is gas carburizing. By diffusion of carbon into the surface and quench hardening, the process produces a strong hard surface layer of martensite of up to 10 millimeters. This thermochemical method adds defined quantities of carbon to the workpiece by us
9、ing a carbon enriched gas (i.e. methane (CH4) or propane (C4H8). After carburization, the components are hardened and tempered to the required surface hardness and to relieve internal stresses. In addition to a high surface hardness (max 850 HV) and abrasion resistance, the heat-treated workpieces e
10、xhibit good reverse bending and fatigue strengths due to residual compressive stress. Specific time and temperature variations in the carburizing, hardening, and tempering processes can be introduced to optimize the material properties and minimize the changes in dimensions associated with the respe
11、ctive charging techniques - hereto lies the art of hardening. Nitriding treating temperatures range from 500C - 580C for gas nitriding and from 400C and up for plasma nitriding and plasma nitrocarburizing. Nitriding is a method for enriching the surface layer of ferrous materials with defined quanti
12、ties of nitrogen or, in the case of nitrocarburizing, of nitrogen and carbon. This not only enhances the hardness, but also the abrasion resistance, fatigue strength, corrosion resistance and antifrictional properties. Furthermore, there are no microstructural transformations from austenite to marte
13、nsite, so high dimensional stability is ensured. Normally, nitriding penetrates to a maximum depth of 0.8 mm. “Profundinieren”, a deep nitriding method developed by Dr.-Ing. Helmut Reese, penetrates to depths exceeding 1.0 mm, depending on the material. Provided that the corresponding steels are use
14、d, non-deforming nitriding is in many instances a viable alternative to case and surface hardening. Nitriding steels are listed under DIN 17211 and EN 10085. Figure 1. The three important surface hardening methods from left to right are case hardening, nitriding, and induction-flame-hardening respec
15、tively 4 13FTM22 Surface hardening is carried out at treating temperatures 50C 100C above the material-specific hardening temperature. The heating can be done by flame, induction, laser- or electron beam. These processes produce a hard surface layer of martensite. These methods can also be used to h
16、arden large components or complex geometries. Induction or flame heating is applied to the heavily loaded areas (specific surfaces) of the workpiece until the respective hardening temperature is reached, after which the workpiece is quenched. Much experience is necessary for optimizing the methods a
17、nd finding component-based solutions for both flame and induction hardening. Much experience is necessary for optimizing the methods and finding component-based solutions for both flame and induction hardening. Therefore the evaluation and consistency of test samples is essential and is greatly enha
18、nced by specific definitions of machine parameters. In summary, surface hardening is a technical and economical alternative to conventional case hardening in many instances. A side-by-side comparison of the three surface hardening methods can be seen in Table 1. The choice and selection of the harde
19、ning method For the load capacity of a component, the important factors are hardness, case depth and core strength: If the load capacity of the gear is vital, case hardening is the first choice, even if the hardening distortion during nitriding is less. If Hertzian pressures are low, as in hydraulic
20、 cylinder applications, and low hardness depths are sufficient, nitriding is the first choice. Large hardness depths in a short time, partial hardening of large components, and flexibility are assets of the surface layer hardening The load capacities of the three hardening methods can be compared us
21、ing Figure 2 where fatigue strength is measured against hardness. Because of the highest load capacity, case hardening is the first choice for the treatment of large transmission components. Table 1. Comparison of three surface hardening methods (pros and cons) Case hardening Surface hardening Nitri
22、ding Pros Large case-hardening depths (CHD) of up to 10 mm, which can be achieved within treatment times of 10 to 200 h Very hard surface layer Very tough core Excellent fatigue resistance by surface compressive stresses Good under impact stresses High surface pressure High surface hardness depths (
23、SHD): 230 mm is possible, which can be reached in a short time of treatment with a relatively low energy consumption. Hard surface layer Tough core Good fatigue strength Good bending and torsion resistance Low treatment temperatures result in minimal dimensional and shape changes and therefore rewor
24、k is seldom necessary Limited accessible areas can be hardened very hard surface layer, depending on the material Tough core Good fatigue properties Improved corrosion resistance Very thermostable Improved sliding properties Cons Due to the high processing temperatures, martensitic transformation an
25、d quenching, considerable dimensional changes and hardening distortions can occur. Not to be used above tempering temperature Little corrosion resistance Hardness depths below 2 mm difficult Only with inductor or flame can easily accessible areas be hardened Increased risk of cracking Expensive samp
26、le hardenings for reproducible results Low nitriding depths with a relatively long treatment time, NHD by definition only 50 HV above core hardness = 1mm NHD is considerably less than 1mm in CHD Under excessive stress, crack and fracture risk due to high surface hardness 5 13FTM22 Figure 2. Load cap
27、acities of the three hardening methods: fatigue strength vs. hardness (Niemann/Winter Maschinenelemente) The main aspects for case hardening of big gear components Three main aspects have to be considered: Hardenability (material selection and geometrical influence) Weight (as much as necessary, as
28、little as possible) Dimensional changes and distortion Hardenability Large components are normally quenched in oil. Cooling below the alloy Ms temperature through to the core often takes longer than 1 hour depending on the size of the cross sections. During this time, the heat is conducting through
29、the surface. Therefore, a steel must be selected, with its ferrite/pearlite nose as far to the right as possible. Otherwise, there will be a significant decrease of the surface hardness and case depth. The larger the cross-sections, the more it is necessary to utilize high alloyed steels. According
30、to the experience at Reese, for large components with a cross section more than 100 mm, 18CrNiMo7-6 or 18CrNi8, or similar steel qualities with HH-alloying-scatter-band, must be chosen. The two time-temperature-transformation diagrams, Figures 3 and 4, exhibit the differences between a high alloyed
31、and low alloyed case hardening steel. The alloying elements Cr, Ni, Mo, Mn and V increase the hardenability, and V, Ni and Mo additions increase toughness. If a steel is selected with insufficient hardenability, regardless of intensive quenching in oil with very good cooling and circulation, the res
32、ult will be an unacceptable drop of hardness and hardness depth of large components. This hardness reduction is evident in gear parts especially below 6 13FTM22 the pitch circle down to the tooth base to a hardness of partially well below 52 HRC. The components can fail after a short period of time
33、by pitting, flank fractures and tooth root fractures. Using the same alloyed steels from Figures 3, 4, 6 and 7, display the relationship of hardness as a function of the distance from the quenched end. As seen, there is an obviously much larger decrease in hardness as the distance between the quench
34、ed end grows with the low alloyed case hardening steel. The graphs created are a result of the Jominy hardenability test (Figure 5). Figure 3. Time-temperature-transformation (TTT) diagram for the high alloyed case hardening steel 18CrNiMo7-6 Figure 4. TTT diagram for the low alloyed case hardening
35、steel 16MnCr5. 7 13FTM22 Figure 5. Jominy hardenability test Figure 6. Hardness as a function of distance from the quenched end for 18CrNiMo7-6 Figure 7. Hardness as a function of distance from the quenched end for 16MnCr5 In the area of decreasing hardness to the tooth root, the CHD cannot be adequ
36、ately modeled by the cylindrical geometry of a sample coupon. In particular, the geometric conditions and their influence on the cooling rates in massive components cannot be compared with small cylindrical test pieces. Case hardening results of the tooth base and tooth flank in comparison with a sm
37、all cylindrical sample coupon are not a sufficient match of the carbon depth profile. The coupon sample should therefore be shaped such that it, in addition to a custom size, also has a greater geometrical resemblance to the tooth base. A 8 13FTM22 comparison of flank and root-CHD and CHD of a 35 mm
38、 sample coupon can be seen in Table 2. A-F represents 6 companies that participated in a collaborative study FVA 501. Steel hardenability has a big influence on shape changes (shrinking and growing). Case hardening steel plants with modern computer controlled melting can specifically adjust the hard
39、enability. It makes sense to require hardenability, grain size and purity. The cost is neutral for the specification for the lower 2/3 or the upper 2/3 band of hardenability. Even more favorable for the subsequent shape change, but subject to a cost surcharge, is the agreement about closer hardenabi
40、lity limits. Figure 8 displays possible shape change values when using case hardening steel as a function of the hardenability. In conclusion, when selecting the material for large gear components, the geometrical influence must be taken into consideration and the following objectives must be achiev
41、ed: 1. A sufficient hardenability of the material must be chosen 2. The material cross sections must be kept as small as possible 3. The geometry must ensure dimensional stability. Also: Grade steel (not “quality” steel) according to EN 10020 Electromagnetically stirred and cooled with fan Fine grai
42、n steel Pre-hardening and annealing in oil or emulsion to estimate dimensional changes Table 2. Comparison of flank and root-CHD and CHD of a 35 mm sample coupon A Flank/30, mm B Flank/30, mm C Flank/30, mm D Flank/30, mm E Flank/30, mm F Flank/30, mm CHDmin 2.75/2.03 3.14/2.20 3.35/2.40 3.05/2.06 3
43、.35/2.28 2.97/2.28 CHD35Coupon4.32 3.43 3.45 3.19 3.50 3.53CHDroot/CHDflank 0.74 0.70 0.72 0.68 0.68 0.77 CHDflank/CHD350.57 0.91 0.97 0.95 0.96 0.84CHDroot/CHD350.47 0.64 0.69 0.65 0.65 0.65 Figure 8. Shape changes when using case hardening steel as a function of the hardenability. 9 13FTM22 Weight
44、 The weight is critical in the first line for efficiency. Weight reduction should be sought in any case, but this must not be at the expense of stability. For large gear components weldments have been proven to be efficient and are now standard. Small and medium sized gears, as seen in Figures 9 and
45、 10 are usually made solid. Larger gears are usually designed as welded constructions The welds must be protected against carburization to avoid cracking during case hardening. See Figures 11, 12 and Figure 13 for examples of larger gears containing weldments. Welding is limited however, by maximum
46、preheating temperature of 350 - 400 C. For large gear components weldments are the design of choice today. They have good dimensional stability and reduce weight. Figure 9. Medium gears that have been made as a solid structure Figure 10. Smaller gears made as a solid structure, no weldments necessar
47、y 10 13FTM22 Figure 11. Example of a larger gear designed as a welded construction Figure 12. Example of a larger gear designed as a welded construction Figure 13. Rim materials, e.g., 18CrNiMo7-6, hub and rim materials e.g., C35, C45, 34CrMo4, 42CrMo4, 36CrNiMo8 Pretreatment of the material Accordi
48、ng to our experience the following pretreatments have proven effective for big gear components and should be examined in detail on costs and benefits: 3-D forging (stretching and compressing with deformation to the core), leads to a more uniform part, reduction of segregation, reduction of pores, an
49、d refined structure to the core. 11 13FTM22 Do not air cool after forging, but let the part temperature drop to 840C. Then soak and harden in oil followed by tempering at 650C. If this is not possible, then in addition to the forging shop, pre harden and temper at or higher that 840C then oil quench and temper at 650C to improve the toughness. One can draw conclusions about the dimensional change and distortion behavior by measuring before and after this process. Avo
