Fig. 1. Influence of heat-treating process on the gear-tooth hardness profile[1]

A wise man once said that anything worth doing is worth doing right. In the case of carburizing, this is especially true since, from both an engineering and a heat-treating perspective, we often take the process for granted – a dangerous precedent that can get us in big trouble. It’s time to review the basics. Let’s learn more.

Carburizing, whether performed in atmosphere or vacuum, is of critical importance to the performance of a given product. For example, in the case of gears, the method of heat treating influences properties (Fig. 1) while carbon content and its distribution in the carburized case affects such engineering properties as:
  • Strength (static and dynamic)
  • Toughness
  • Pitting resistance
  • Case-crushing strength (to determine minimum case depth)
  • Wear resistance
  • Sensitivity to cracking
  • Grinding burns
  • Operating life
While the effective case depth (ECD) of a gear is often measured as 50 HRC, the depth of high hardness – generally considered >58 HRC – is a major contributor to the improvement of the properties listed above. Other considerations in the production of a quality gear include microstructure (tip, active flank, root), core hardness at the center of the tooth, hardness at the tooth flank and root, effective case depth (root fillet), and case-depth variation (flank-to-root).

The Importance of Specifications

In most organizations, the engineering department selects the material and specifies the properties required to satisfy a given customer’s product performance demands. They often write the heat-treat specifications as well. The interpretation of these specifications, however, is often left to the heat treater, who must be aware of his equipment limitations, operating condition and current maintenance state.

Well-written specifications serve a number of important purposes.
  • To provide a way to convey the desired properties (e.g., mechanical, physical and metallurgical) for a specific material and part number
  • Serving as a set of instructions, often in the form of a recipe for the heat treater to follow
  • To provide a mechanism to capture changes to the manufacturing process (including heat treatment) so that innovations or experience-based procedures are not lost
  • Serving to help the engineer choose the correct heat-treatment method and communicate it with less drawing clutter

Fig. 2. Example of improper carburization – soft spots. (a) Nital etched bearing journal revealing irregular white patch. (b) Bearing-journal cross-section revealing area of no carburization.

Elements of a Good Specification

Too often specifications do not reflect what is actually being done. We forget to fully document the entire heat-treatment process, including the anticipated distortion state after heat treat and what must be done to compensate for it (e.g., stock allowances, straightening procedures, post-machining methods). In addition, a feedback step MUST be included so that changes can be captured into the document. Working together, the engineer and heat treater should call out the following as a minimum:
  • Scope
  • Application (including allowable tolerances)
  • Drawing reference
  • Pre-processing steps
  • Pre-cleaning
  • Stress relief (if necessary)
  • Loading (configuration, fixturing, net and gross loading)
  • Carburizing (temperature, time, carbon potential)
  • Hardening (temperature, time)
  • Tempering (number, temperatures, times)
  • Deep freezing (if required)
  • Inspection methods (e.g., hardness, NDT, etc.)
  • Post-cleaning
  • Straightening (if required)
  • Post-processing steps
  • Metallurgical checks
  • Rework allowances
  • Records and reports
  • Manufacturing feedback (e.g., heat treating)
The application should detail the desired microstructure (e.g., martensite percentage, allowable non-martensitic transformation products, carbide type and distribution, allowable retained-austenite percentage, etc.) as well as required testing (e.g., case depth, surface and core hardness, Charpy values, etc.). Important properties such as rolling-contact-fatigue values and maximum bending stress for gearing should also be identified.

Testing methods and locations should be clearly identified (Fig. 2) and the type of test samples specified. Sampling methods, anticipated values (e.g., oxygen-probe millivoltage, three gas-analyzer readings, dew-point values) and confirmation methods such as shim stock, turn bars, spectrographic-analysis coupons and fracture bars should be fully detailed. The failure to provide complete information means that the heat treater must try to anticipate these types of requirements, which may unintentionally introduce variables into the process.

Communication between engineering and the heat treater or between engineer and supplier is critical to success. The excuse “he should know what to do” just isn’t part of an acceptable quality plan today.

Material Selection

A number of important factors need to be considered when choosing a material for carburizing, not the least of which is the steelmaking process, chemistry, trace elements (and their percentages), inclusions (type and distribution), material form, hardenability (Jominy, DI), grain size and mill treatments. It is especially important to know the source of the raw material (e.g., foreign or domestic source, mill heat or mixed lot). These elements can be divided into the following general classes:
  • Design aspects
  • Environment
  • Metallurgical requirements
  • Manufacturing needs
For example, manufacturing must be concerned about material form, cost, availability and quantity, heat treatment, inspection, equipment availability and condition, familiarity with the material in question, and machinability to name a few.

Fig. 3. Example of carburizing simulation software (Courtesy of Super Systems, Inc.)

How to Achieve Total Control

Today, whether atmosphere or vacuum carburizing, a number of simulation packages (Fig. 3) can be employed to ensure repeatability and documentation of results. These systems not only provide for real-time monitoring and control of results, but they allow “what if” scenarios to aid in the establishment and refinement of process cycles.

Despite these control measures, more and more companies are requiring the sacrifice of one or more parts in a load for complete metallurgical work-up and mechanical testing. This procedure – once almost exclusively restricted to aerospace – has become commonplace in many automotive and commercial heat-treat shops. One caution, however, is to select parts from representative areas within the load, not just those that are convenient to the operator. Predetermining where the part is to be taken from in the load assures that results truly represent the heat treatment that has taken place. IH