In this article we will examine the relationship between ultimate tensile strength, fatigue strength, and elongation versus the depth of carburization. The data for this comparison was taken from several iterations discussed in previous articles and shown in Table 1. Depending on the data set, the depth of carburization (case depth) is defined as the distance from the surface to 50 or 58 HRC, expressed as a percent of the 0.200 inch gage diameter.
The first set of data is for the simulated carburized steel core samples (Table 1: Carb Core) with a hardness of approximately 45 HRC. This data set is made up of iteration #39 (SAE 8620 steel), iteration #82 (SAE 86B20 steel), and iteration #130 (DIN 20MnCr5 steel). These were the only three sample iterations with this core hardness, which was chosen to be consistent with the next data set.
The second data set is the simulated case-core composite samples (Table 1: Light Case) from a previous article (Carburized Case-Core Samples). The iteration numbers were #62 (SAE 8620 steel), #63 (4320 steel), and #70 (8620 steel). These samples were carburized with an effective case depth to 50 HRC that was 5% (#63) or 10% (#62, #70) of the diameter. The core was hardened to approximately 45 HRC.
The third data set was the early through-carburized samples (Table 1: Thru-Carb Early) that were typically ground and polished after heat treatment. These were iteration numbers #38, #48, #50, #54, #56, #58, #60, and #71. The steels were made up of SAE grades 8620, 4620, 4320, 5120, 9310, and 8822. These samples were carburized at 927 °C for 24 to 26 hours. The surface hardness was typically above 60 HRC and the core hardness was above 55 HRC. The depth of carburization to 58 HRC was approximately 37.5% of the diameter.
The fourth data set was the through-carburized samples (Table 1: Thru-Carb Late) that were not ground after heat treatment. These were iteration numbers #73, #75, #79, #83, #87, #88, #89, #90, #95, and #137. The SAE grades used were 41B17, 86B20, 8620, and DIN 20MnCr5. The heat treatment cycle, hardness and carburization depth were similar to the third data set.
The last data set was the through-carburized samples (Table 1: Actual Thru-Carb) that were carburized completely to the center of the bar. These were iterations #40, #41, and #141. The first two iterations were SAE 8695 steel and the third was DIN 20MnCr5 steel carburized at 927 °C for 36 hours.
A graph of the ultimate tensile strength and fatigue strength versus the carburized depth is shown in Figure 1. By carburizing to 5 to 10% of the gauge diameter, the average ultimate strength increases from 1507 MPa with no case to 1715 MPa. By increasing the case to approximately 37.5% of the diameter, the average ultimate strength decreases to 1667 MPa for the samples ground and polished after heat treatment, and 1525 MPa for those not ground and polished after heat treatment. This indicates there is a benefit to grinding and polishing after heat treatment by removing intergranular oxidation. By completely through carburizing the gage diameter, the average ultimate strength further decreases to 1147 MPa. For these axial test bars, a small amount of high carbon case depth is beneficial for ultimate strength. However, increasing the carburization depth too much can significantly decrease the ultimate strength, which supports the practice in industry to develop components with a shallow case depth to increase properties.
Figure 1 also shows as the depth of carburization increases so does the spread between the minimum and maximum tensile and fatigue strength. However, data sets three and four, with the largest spread, had eight and ten samples respectively, while all other data sets had only three samples. More data would be necessary to confirm this observation. The fatigue strength follows the same trend as the ultimate strength, and the amount of spread appears proportional.
A graph of the elongation versus the carburized depth is shown in Figure 2. The core samples with no carburized case depth (Carb Core) have the highest average elongation at 29.6%. Carburizing to 5 to 10% of the diameter decreased the average elongation to 16.2% (Light Case). Carburizing to 37.5% of the diameter further decreases the average elongation to 3.7% (Thru-Carb Early) and 2.4% (Thru-Carb Late). Through carburizing the samples to 50% of the diameter resulted in an average elongation of 1.7%. The maximum amount of spread was in the core samples at the far left side of the graph.
In conclusion, carburizing these axial test bars through the diameter provides the lowest ultimate and fatigue strength, and the lowest ductility. The maximum combination of ultimate strength, fatigue strength, and ductility is obtained by carburizing to 5 to 10% of the gage length diameter; further validating industry practice.