Reproducibility and Process Control
Reproducibility of metallurgical processes demands the definition of:
- Material specification
- Engineering requirements
- Metallurgical process specification
- Quality assurance
Material SpecificationAnalysis of the steel must be reasonably consistent. This necessitates more than just reading the accompanying test certificate. The test certificate contains valuable information that embraces not only the physical chemistry but also the mechanical properties such as hardness, tensile strength, grain size, surface decarburization and hardenability. These are all properties that can be checked by the receiving department to ensure consistency of steel prior to issuance for manufacturing.
Engineering RequirementsThe engineering requirements must consider the suitability of a choice of steel for the part application and its operating environment. This is a fundamental requirement and should be coordinated with the incoming material testing results.
Metallurgical Process SpecificationIn addition to material and engineering requirements, a number of process parameters need to be addressed in order to achieve repeatable metallurgy. These include:
- Part cleanliness prior to heat treatment – This is the start of the metallurgical processing procedure. If the part is contaminated with cutting oils, cutting fluids or grease when it enters the furnace, the work surface will be contaminated during the metallurgical procedure. This could lead to soft spots, and it can also contribute to intergranular corrosion. Part cleaning should be the initial procedure or point of control for all heat treatment.
- Process temperature selection – Too high of an austenitizing process temperature can result in grain growth, and if the part has to be quenched from the high process temperature, retained austenite can result.
- Process time at the selected temperature – Exceeding the selected time at temperature will also contribute to grain growth. With the process of carburizing, extended times will occur at the selected process temperature. This will tend to initiate grain growth in carburizing steel.
- Process temperature uniformity – Temperature uniformity within the process chamber is mandatory, particularly if conducting an aluminum-alloy solution treatment. Temperature uniformity is critical in all aspects of heat treatment, no matter what the process procedure is.
- Atmosphere and control – Atmosphere composition needs to be consistent and precise for each cycle. Atmosphere control for processes such as nitriding, ferritic nitrocarburizing, austenitic nitrocarburizing, carbonitriding, neutral hardening and salt-bath analysis (if salt-bath heat treating) is critical to monitor the atmospheric process conditions. Adjustment needs to be made either manually or electromechanically.
- Quench tank control – Maintaining the quench medium at the temperature that satisfies the metallurgical requirements of the parts and cleanliness of the quench medium (metallic fines, carbon in the form of soot and other foreign matter) is key.
- Tempering temperature control – It is this portion of the metallurgical process procedure that is perhaps most important because it will determine the final mechanical strength of the treated part. This is, of course, providing that temperature uniformity is accurate and better than +/-10°F.
- Hardness testing – It is necessary to ensure repeatability and consistency of hardness testing. At least a daily check of the hardness consistency and repeatability of the appropriate machine by checking with a standard known-hardness test block in accordance with ASTM specification E-18 A, B, C or D is required. It is recommended that a daily test or a shift test be conducted before any work is inspected on the hardness test unit.
Quality AssuranceQuality assurance of a metallurgically processed workpiece commences with the quality and consistency of the steel analysis. From that point, all aspects of the process can be monitored and maintained to ensure repeatable, consistent metallurgical quality.
This column is sponsored by Praxair Technology Inc.