Often, the heat treater is confronted with low properties (hardness or tensile properties) in a part or heat-treated load. Determining the root cause of the problem can be difficult and bewildering. This article will provide some direction regarding understanding some of the root causes of low properties.
Introduction
There are many reasons why a part can fail to meet properties. Some of these reasons are obvious, and some are quite subtle and difficult to detect. The purpose of this article is to help the heat treater understand the sources of low hardness and correct them.
In general, sources for inadequate properties can be divided into several different categories as illustrated in Figure 1.
Material
Improper or incorrect material that is heat treated is one of the most common problems facing the heat treater. The material could be the wrong alloy, not meet specifications, or have inadequate hardenability to achieve the desired hardness with the required quenchant. Another possibility is that the material is correct as specified on the engineering drawing, but unable to achieve the desired properties that the designer has specified on the engineering drawing.
The first thing to examine is the incoming material certifications. The alloy and chemistry should be specified, and depending on the vendor or procurement specification, the Jominy hardness and the Ideal Diameter (DI) based on chemistry should be provided. Knowing the chemistry and Jominy (or DI), the expected as-quenched hardness can be calculated for the quenchant [1][2].
If the hardness calculated is near the expected or specified hardness, then the problem can either be incorrect material or something wrong with the process, furnace, or quench system. One way to isolate whether it is incorrect material or the processing is to take the same part from the same heat lot and heat treat it in another, similar furnace with the same quench oil. If the part achieves the desired properties in the other furnace, then it is likely that the material is correct, and that the problem lies with the furnace or quench system. If the part does not achieve properties, then it is likely that the material is incorrect.
To verify the material, it is necessary to send the material to an outside and independent laboratory for spectrographic analysis. The cost of this analysis is often nominal and is useful evidence should any disagreements occur with the part supplier or steel distributor.
Process
The typical heat-treating process for a steel component is shown in Figure 2. Detailed specifications for heat treating a given steel are often provided in many different books [3] and process specifications such as AMS 2759 [4].
During austenization, the part is heated to approximately 25-50°F (14-27°C) above the A3 temperature on the phase diagram in the austenite phase field for the alloy. Should the part not achieve this temperature, the part will be in the ferrite + austenite phase field and not convert completely to austenite. Upon quenching, the part will not achieve the proper hardness, and the microstructure will consist of ferrite and islands of martensite.
It is also important to have adequate time above the A3 temperature to allow all the ferrite, pearlite, and carbides to dissolve in the matrix. This is not instantaneous, as it takes time for the constituents to dissolve into the austenite. The old rule-of-thumb of “one hour per inch thickness” is conservative and will provide consistent results.
To successfully heat treat or carburize the many different alloys seen in the heat-treating shop, it is necessary to properly control the carbon potential of the atmosphere. In neutral hardening, this means that the carbon potential is neutral to the part and is neither carburizing nor decarburizing. In carburizing, this means that the desired surface carbon content is achieved so that the desired case depth is achieved.
Regardless of the source of the atmosphere, whether from an endothermic generator or generated in-situ with nitrogen-methanol, it is important that the carbon monoxide, CO, content be maintained at 20% (23% in the case of propane). If the carbon monoxide percentage is not maintained at 20%, then the oxygen probe will not read the correct carbon potential. If propane is used, then the carbon analyzer must be adjusted to reflect the percentage of CO from propane (23%). All the charts and graphs used to determine carbon potential from dewpoint are based on a carbon monoxide content of 20%. The CO2 concentration will vary depending on the desired carbon potential.
In addition, for neutral hardening, the carbon potential must match the activity of carbon in the steel [5] [6]. However, the carbon activity coefficient is also dependent on the alloying content of the steel [7]. For the most part, the effective carbon potential is higher than the carbon content in the steel. In some steels, the effective carbon potential is slightly lower than the carbon content. In these cases, it is best to use the carbon content in the alloy.
The presence of decarburization can give the appearance of low properties. It is important, when harness testing a part, that the surface is slightly abraded to provide a scale-free surface. Should the surface measurement be soft due to decarburization, additional material removal may be necessary. At worst, a part should be sectioned and the hardness taken below the surface to verify hardness. Lastly, a metallographic specimen could be taken to measure and verify the depth of decarburization.
Conclusions
In this first column in the series, we discussed the methods of determining some of the sources of inadequate properties due to material and process. In future columns, we will discuss the effect of potential problems associated with the furnace and quench system that can cause low properties.
Should you have any questions or comments regarding this article, or have suggestions for further articles, please contact the editor or the author.
References
- D. S. MacKenzie, “Determining Grossman H-Value from cooling curve data,” Thermal Processing, no. February, pp. 21-23, 2020.
- D. S. MacKenzie, “Predicting hardness by the Grossman H-Value,” Thermal Processing, no. March, pp. 12-13, 2020.
- American Society for Metals, Heat Treaters Guide Standard Practice and Procedures for Steel, Metals Park, OH: ASM International, 1982, pp. 423-426.
- SAE International, AMS 2759G, Heat Treatment of Steel Parts, General Requirements, Warrendale, PA: SAE International, 2018.
- R. Collin, S. Gunnarson and D. Thulin, “A Mathematical Model for Predicting Carbon Concentration Profiles of Gas-Carburized Steel,” Harterei-Tecn Mitt., vol. 25, pp. 17-22, January 1970.
- R. Collin, S. Gunnarson and D. Thulin, “Mathematical Model for Predicting Carbon Concentration Profiles of Gas-Carburized Steel,” Journal of the Iron and Steel Institute, vol. 210, no. 10, pp. 785-789, 1972.
- T. Ellis, I. M. Davidson and C. Bodsworth, “Some thermodynamic properties of carbon in austenite,” Journal of the Iron and Steel Institute, vol. 201, pp. 582-587, 1963.
- L. Rothleutner, “How to keep decarburization in check,” Thermal Processing, no. May, pp. 20-21, 2019.
- D. S. MacKenzie, “Calculating decarburization,” Thermal Processing, no. July, pp. 22-23, 2023.
