Manufacturers can benefit greatly by using gear steels that provide consistent quality and can minimize their manufacturing costs. The next few articles in the Materials Matter series will focus on steels that optimize both of these gear steel attributes. This includes discussion of gear steels that deliver consistent heat-treat response, are optimized for use in vacuum carburizing operations, offer good machinability, and reduce distortion issues.
Heat Treat Response – Steelmaking Influences
During steel gear manufacturing, gears are heat-treated to help produce the final mechanical properties essential for part performance. Common heat treatments include carburizing, carbo-nitriding, nitriding, and induction hardening. Key to the consistent heat-treat response is the raw material properties, especially the steel chemistry.
Many factors must be considered to select an appropriate steel grade for a given gear application [1]. Once a steel chemistry grade is selected that matches the requirements of the specific gear cross-section and heat treatment involved, consistency in steel chemistry is a significant factor in consistent heat-treat response.
Strict controls of chemistry and hardenability aims are necessary to minimize heat-to-heat variation, especially if the heat-treat parameters are tuned to a specific part geometry. Figure 1 shows the standard hardenability range of a common carburizing gear steel, 8620. State-of-the-art steelmaking techniques, including ladle-refining processes, allow very tight chemistry control. With that tight chemistry control, a much tighter hardenability range can be guaranteed as shown by the 8620H band.
During steelmaking, control of nitride and carbide forming elements such as aluminum, titanium, and vanadium are essential for prior austenite grain size (PAGS) control. A fine PAGS (ASTM Size 5 or finer) is desired to maintain a consistent steel hardenability effect and for optimum fatigue properties [2].
Consistency of chemistry throughout the heat of steel starts with good ladle-refining practices. Most modern steelmaking shops include ladle-refining stirring techniques either with gas injection and/or induction stirring that maintains both chemistry and temperature homogeneity prior to casting. Rapid analytical techniques (atomic emission spectrometers) coupled with these ladle-refining techniques give steelmakers the ability to control most significant alloying elements to within ± 0.01 weight percent.
Once chemistry control is achieved during the ladle-refining process, the casting (solidification) process must be controlled to minimize chemical segregation. During solidification, some of the chemical elements in steel will partition or segregate between the growing solid crystals and the parent liquid. This results in segregation of those elements typically in the centerline of continuously cast products, and at the tops and bottoms, as well as centerlines, of ingots [3], Figure 2. It is the role of the steelmaker to minimize these effects by using good casting practices and good ingot mold design.
A final steelmaking process that will influence the chemical variation within the steel is the hot deformation (e.g. forging, rolling) to convert ingot or bloom into the final raw material form. With a reasonable degree of hot work, solidification shrinkage is effectively eliminated and dendritic crystals are broken up and recrystallized. Inclusions cannot be removed by hot work, but may be changed in size, morphologies, and distribution by hot work [3].
Heat Treat Response — Gear Heat Treating Methods
The design/application service requirements for demanding gear applications require that the gear teeth surfaces are case hardened to a specified depth by a final heat-treating operation of the gear teeth, or entire gear profile in cases where the core properties are also elevated. The heat-treatment methods used in the industry for steel gears include 1) carburizing/carbonitriding (gas or vacuum), 2) induction surface hardening, and 3) nitriding (in its various forms), and each method has specific base steel requirements to ensure that the final gear attains the necessary performance level.
Carburizing: Carburizing (and its variants) has the advantage that a lower carbon base material (readily manufacturable) can be applied to the gear preform, and the heat-treat response is then controlled by the alloying composition of the base grade of steel. As such, the selection/design of the base grade must correspond to the quenching method used for hardening of the gear following carburizing/austenitizing to achieve the specified case and core properties or depth.
Induction Hardening: Induction skin/surface is applied to a higher carbon base grade (to achieve the high surface hardness requirements after hardening), where the properties of the gear preform become the core properties of the final gear application. The induction surface heating parameters/process is designed to rapidly reach steel austenitizing temperatures to a specified depth, followed by a rapid quench to harden that surface layer. For shallower case depth gears, the heat-treat response of this rapid operation is more dependent on the dissolution of the prior structure into the austenitic state than the measured quench hardenability of the steel. As such, the steel composition and prior process history (hot rolled, normalized, quenched and tempered, etc.) that dictates the chemistry/structure going into the induction heat treating operation becomes vitally important to achieve a high ratio between the induction heated depth and the actual hardened depth. In general, a finer prior grain/carbide structure of the base material results in an increase in this ratio, and a more desirable response to meet both the metallurgical and dimensional requirements of the final gear application.
Nitriding: Nitriding (in its various forms) has the distinct advantage of not requiring an austenitization and quench cycle to be applied to the gear preform, therefore minimizing the distortion that naturally occurs during the quenching and phase transformation process. However, this ferritic-based process develops a much shallower case depth (for a given cycle time), and the case hardness and depth are dependent upon the composition and structure going into that process. The subcritical diffusion of nitrogen into the gear surface results in the formation of fine nitride particles that both strengthen and alter the stress state of the surface layer, thereby increasing the fatigue properties of gear teeth. The nitride particles form from various alloying elements contained with the base steel grade, each one having a different effect on the case properties developed. In addition, the state of these elements (in solution in the iron matrix or tied up as nitrides or carbides and size of the particles) dictates the effectiveness of nitriding operation (case hardness and depth achieved) applied to the gear application. It should also be noted that the core properties of the gear preform are typically stabilized by a prior temper operation (approximately 50° F higher than the nitriding temperature), which then set the core properties of the final gear application.
In conclusion, many factors need to be considered in the design and manufacturing of gear components to optimize quality and minimize costs. The selection of steel grade and quality coupled to the corresponding heat treat process are foundational steps.
References
- Engineered Gear Steels: A Review, C. V. Darragh, Gear Technology, Nov./Dec. 2002
- Steel – Processing, Structure, Performance, Second Edition, Chap. 16, G. Krauss, 2015.
- Solidification, Segregation, and Banding in Carbon and Alloy Steels, 2003 Howe Memorial Lecture, G. Kraus.
