Pyrowear® Alloy 53 steel is an excellent material for the construction of drive transmission components (gears, shafts, etc.) that must operate in difficult conditions, primarily in the aviation industry (Fig. 1). Among the properties of such items is high case hardness and abrasion resistance, while the core remains flexible and is capable of carrying large impact loads. They can work at elevated temperatures with limited lubrication. The thermal treatment of such materials is based on case hardening by carburizing.

This article presents the vacuum-carburizing technology of Pyrowear® Alloy 53 steel. It discusses the methods of establishing different carbon concentration profiles in a layer and their influence on the hardness profile. The effects of subzero treatment and tempering on the properties of the hardened case are also considered.

It also presents a SimVaC® simulator of vacuum carburizing, designed especially for Pyrowear Alloy 53 steel, which provides highly accurate predictions of theoutcome of the process (e.g., profile of carbon concentration in the layer or vice versa) by establishing the process parameters that will guarantee the required profile.

Properties of Pyrowear® Alloy 53 Steel^[1]

The steel is produced by Carpenter Technology Co., and it is intended specifically for use in heat-resistant drive units operating at temperatures exceeding the operating temperature of AISI 9310 steel. The chemical composition of the steel is as follows: 0.10% C, 0.35% Mn, 1.0% Si, 1.0% Cr, 2.0% Ni, 3.25% Mo and 2.0% Cu. Two additional alloys – molybdenum and copper – distinguish it from commonly used steels for carburizing. Molybdenum increases the heat resistance and improves resistance to abrasion wear, whereas copper improves the resistance to shock load and lubrication properties. This composition of alloy additives determines the phase parameters of steel (Table 1), which in turn determines the temperature range of the thermal treatment.

The thermal treatment of Pyrowear Alloy 53 steel involves the formation of a hardened case by carburizing and hardening (case hardening). The following are the phases of the process and the temperature parameters recommended by the producer.

1. Carburizing at 870-930°C (1600-1700°F)

2. Oil or gas quenching from 905-920°C (1660-1700°F)

3. Subzero treatment below -75°C (-100°F)

4. Tempering at 150-290°C (300-550°F)

Sometimes subcritical annealing at 600°C (1100°F) is applied. A properly conducted thermal treatment results in a hardened layer with the case hardness exceeding 60 HRC. Core hardness is approximately 35 HRC, strength is approximately 1,200 MPa and impact resistance (V-notch) is approximately 100-150 Joules.

Test Procedure

The processes were conducted under industrial conditions in a standard SECO/WARWICK 15.0VPT-4035/36IQCN vacuum furnace (Vector line) with a working space of 600 x 600 x 900 mm (24 x 24 x 36 inches). An LPC carburizing system was used, which also enables high-pressure gas quenching (Fig. 2). The parts under study made from Pyrowear Alloy 53 steel were placed together with a ballast charge of approximately 250 kg (551 pounds), which reflected the standard industrial process conditions. The whole process ran according to the following sequence:

1. Vacuum carburize in the FineCarb® technology^[2]

2. Slow cooldown followed by annealing and continued cooling to the ambient temperature

3. Heat-up for quenching and high-pressure nitrogen quenching^[3]

4. Subzero treatment in a cryogenic chamber

5. Temper in nitrogen

All the processes except the subzero treatment were conducted in the vacuum furnace. However, it is possible to install a subzero treatment system in a vacuum furnace. Then the whole process can be carried out in one device, within one work cycle.

Reference Process

First, a reference process was conducted for the purpose of:

•   Verification of the compliance of the simulation of the LPC carburizing process with the actual results
•   An analysis of the microstructure and hardness profile after consecutive process stages

The aim of the process was to obtain a hardened layer on Pyrowear Alloy 53 steel with a surface hardness exceeding 60 HRC and a surface carbon concentration of approximately 0.95%. The effective case depth (ECD) aim was 1.15 mm (0.045 inch) at a hardness criterion of 50 HRC (for a carbon concentration of 0.31%). A high surface-carbon concentration of 0.95% is a result of the need to achieve the maximum case depth with a hardness exceeding 60 HRC, as measured by the ratio of the 60 HRC case depth and the 50 HRC case depth. Additionally, the amount of residual (retained) austenite should not exceed 20%. Moreover, carbide network along grain boundaries should be lower than 30%. These parameters reflect the typical requirements of the aviation industry regarding this technology.

To meet those expectations, a process was developed consisting of the following phases:

1. LPC carburizing at 980°C (1800°F)

2. Subcritical annealing for 1 hour at 600°C (1100°F).

3. Nitrogen quenching at 12 bar at 910°C (1675°F)

4. Subzero treatment at -90°C (-130°F) for 1 hour

5. Tempering at 230°C (450°F) for 1 hour

A decision was made to conduct the carburizing process at higher temperatures than recommended by the steel producer due to the four-times shorter process duration (the process conducted at the recommended temperatures lasts over 8 hours). The negative effect of the temperature on the growth of austenite grains was offset by annealing.

The LPC process was simulated with the SimVaCprogram, which has the characteristics of Pyrowear Alloy 53 steel in its database. Figure 3 shows the result of the simulation in the form of the process parameters. The final carbon-profile curve is above the curve illustrating the changes of the surface-carbon concentration during the carburizing process.

In order to obtain the required ECD layer of 1.15 mm (0.045 inch) at a carburizing temperature of 980°C (1800°F), the LPC process should be divided into pairs of boost (C) and diffusion (D) according to the following time sequence: C/D = 5/3+4/6+4/8+4/10+4/12+4/16+4/25. The duration of the carburizing process is only 2 hours, 7 minutes. The simulation takes into account the effect of reheating for quenching on the final carbon profile.

The process was conducted in accordance with the assumptions/simulation, and a final case depth of 1.15 mm (0.045 inch) was achieved. Surface hardness was 64 HRC and core hardness was 42 HRC. At the same time, a case depth of 0.76 mm (0.030 inch) was achieved at the criterion of 60 HRC, which accounts for over 60% of the case depth at the criterion of 50 HRC. The case microstructure (Fig. 4) consists of tempered martensite with uniformly distributed inclusions of fine, globular carbides with a size of under 1µm, forming larger structures immediately below the surface (within the requirements range). Residual austenite is present in quantities that cannot be estimated with an optical microscope (below 5%).

The graph in Fig. 5 shows the hardness profiles reflecting the transformations that occur in the layer after subsequent phases of the process. The carburizing process and cooling down slowly resulted in a profile typical of a martensitic structure with a high level of residual austenite near the surface (over 50%). The process of subcritical annealing resulted in the formation of a fine ferritic-austenitic structure with much lower hardness and uniform formations of fine carbides.

Hardening again raises the hardness profile to the previous level, forming a martensitic-austenitic structure with a characteristic decrease in hardness near the surface. This effect, if removed by subzero treatment (austenite to martensite transformation), increases the surface hardness to 65 HRC. The tempering process results in the formation of a stable final microstructure of tempered martensite.

Comparative Processes

In the next stage of the study, additional processes were conducted. Their purpose was to show the effect of a change in the temperature of quenching, subzero treatment and tempering on the case hardness profile. Only one temperature parameter was changed in the processes. Otherwise, it was conducted according to the reference process.

For quenching, the process was run not only at the reference temperature of 910°C (1675°F) but also at higher (955°C/1750°F) and lower (870°C/1600°F) temperatures. The graph in Fig. 6 shows the hardness profiles, which indicate that quenching temperature has the greatest effect on the core hardness. It increases with temperature to 37, 41 and 43 HRC and results from the higher solubility of carbides and higher saturation of austenite with carbon at higher temperatures. The case hardness is very stable and decreases only by 1 HRC (from 64 to 63 HRC) at a temperature of 870°C (1600°F). A higher temperature of quenching (955°C/1750°F) slightly increased the case depth from 1.15 mm (0.045 inch) to 1.26 mm (0.050 inch) as an effect of better hardenability.

The subzero treatment was conducted at three temperatures: -60°C (-80°F), -90°C (-130°F) and -120°C (-180°F). The respective hardness profiles are shown in Fig. 7. Both hardness profiles after the subzero treatment at lower temperatures are nearly identical. The subzero treatment at the temperature of -60°C (-80°F) resulted in a slight decrease in the surface hardness, which indicates a larger amount of residual austenite compared to the processes conducted at the temperatures below -90°C (-130°F).

Similarly, tempering was additionally conducted at a temperature lower (175°C/350°F) and higher (290°C/550°F) than the reference temperature. The relevant hardness profiles are shown in the graphs in Fig. 8. The tempering temperature was not proven to have any effect on the core hardness or the case depth within the temperature range under study. Slight differences in hardness were observed near the surface. It decreased slightly with an increasing temperature (from 65 HRC to 64 HRC down to 63 HRC at a temperature of 290°C/550°F), which confirms elevated thermal stability of the case.

Shaping the Hardness Profile

The requirements for a hardened case frequently contain a parameter that characterizes the shape of the hardness profile. This is defined as the minimum ratio of the depth of two layers for two different criteria, usually with a hardness of 60 HRC and 50 HRC (e.g., minimum 40% of the case depth at the criterion of 60 HRC relative to the case depth at 50 HRC). The effect of the surface concentration of carbon and case depth was analyzed as part of this issue.

The graphs in Figure 9 present simulated (SimVaC) profiles of carbon concentration differing from surface carbon concentration with the same case depth for the adopted hardness criterion of 50 HRC, which corresponded to a concentration of 0.30% C. Another case-depth criterion was adopted for 60 HRC, to which the carbon concentration of 0.70% was assigned. Case depths for each carbon profile were determined at both criteria (60 and 50 HRC), and their ratio was calculated. The highest ratio (70%) of case depth at 60 HRC to that at 50 HRC was achieved at the highest surface concentration of 1.00% C. It decreased gradually with a decrease in surface carbon concentration.

The analysis shows that the ratio of case depth at the higher hardness criterion (60 HRC) to the case depth at the lower hardness criterion (50 HRC) can be changed from 0% to the maximum (practically to 70%) by changing the surface carbon concentration. The maximum surface carbon concentration is limited by the formation of carbides and the limits of retained-austenite formation, which depend on a specification, steel grade and the parameters of thermal treatment.

For the same surface carbon concentration, different criteria showed no effect on the case-depth ratio. Figure 10 shows carbon profiles and the calculated constant case-depth ratio of 50% for all the profiles.

Summary and Conclusions

•   Vacuum carburizing by the FineCarb® method and high-pressure gas quenching (HPGQ) meets the requirements of the case-hardening process for Pyrowear® Alloy 53 steel.
•   The whole process can be conducted in a single-chamber vacuum furnace in one working cycle.
•   Vacuum carburizing processes are simulated with high accuracy on a dedicated SimVaC® simulator, whose database contains Pyrowear Alloy 53 steel.
•   It is justified to apply an elevated carburizing temperature, which reduces the process duration greatly –
for example, four times at a temperature of 980°C (1800°F).
•   The case hardness profile depends mainly on the carbon concentration profile, but it is also affected by the temperature of quenching, subzero treatment and tempering.
•   The temperature of quenching directly affects the core hardness (the higher the temperature, the higher the hardness), which reaches 37-43 HRC at temperatures of 870-955°C (1600-1750°F).
•   The temperature of the subzero treatment affects the degree of austenite transformation. The optimum temperature is -90°C (-130°F).
•   The tempering process affects surface hardness: the higher the temperature, the lower the hardness. A hardness of 65-63 HRC was achieved at temperatures of 175-290°C (350-550°F), which confirms the resistance of Pyrowear Alloy 53 steel to increased operational temperature.
•   Pyrowear Alloy 53 steel can be carburized to a surface carbon concentration level of approximately 1.0%, which makes it possible to obtain a flat profile of high hardness.
•   The ratio of the case depth at a hardness of 60 HRC to the case depth at a hardness of 50 HRC directly depends on the surface carbon concentration and not on the case depth.

For more information:  Contact Maciej Korecki, Ph.D. Eng., SECO/WARWICK S.A., ul. Sobieskiego 8, 66-200 S´wiebodzin, Poland; tel: +48 683820506; e-mail: Maciej.Korecki@secowarwick.com; web: www.secowarwick.com

References:

1. Carpenter Technology Co., “Pyrowear^® Alloy 53 – Technical Datasheet,” 2011

2. P. Kula, M. Korecki, R. Pietrasik, E. Stan´czyk-Wolowiec, K. Dybowski, “FineCarb^® - the flexible system for low pressure carburizing,” 17th IFHTSE Congress, Kobe, Japan, 2008

    3. M. Korecki, J. Olejnik, P. Kula, R. Piertasik, E. Wolowiec, “Multi-purpose LPC+LPN+HPGQ 25 bar N2/He single chamber vacuum furnaces,” 26th ASM Heat Treat Conference and Exposition, Cincinnati, Ohio, 2011