Heat treatment is an inseparable part of many industries. This is especially true of the automotive industry with its emphasis on using the most current work procedures and devices and the requirement for the most effective processes.

Setting up processes such as low-pressure carburizing led to many changes in the previous design of heat-treatment fixtures. The first change is the replacement of common austenitic steels by nickel-based superalloys. Fixture design was also changed with a view to lowest possible weight, leading to much better furnace effectiveness. Close cooperation of the design and research departments was historically the key to success in innovative fields.

    In 1998, when the new technology of low-pressure carburizing (LPC) widely entered the market, Safe Cronite quickly developed the alloy Mancellium, which is extremely resistant to carbon diffusion, which was limiting the lifetime of existing alloys. More recently, in response to the trend to increase the working temperature of the LPC process from 950 to 1050°C (1742 to 1922°F), the company has developed improved alloys.

Carburizing Process Principles in Cr-Ni Steels

Over its lifetime, furnace alloy must resist carbon diffusion. Therefore, it must contain enough carbon-protecting elements, which are most commonly strong carbide-forming elements such as Cr, Ti, Nb and W. Such elements form carbides and effectively inhibit the diffusion of carbon into the internal structure. Chromium also creates a protecting layer of Cr₂O₃ on the surface of the part, which protects against other corrosion environments. Nickel – also present in Cronite alloys – slows the speed of carbon diffusion significantly. Therefore, the alloys containing higher amounts of nickel (e.g., HR32 or HRZ) commonly achieve better total lifetime in terms of carburizing resistance.

    The LPC process brought new challenges for alloys, predominantly due to the impact of the environment on the total lifetime. That led to a second generation of alloys (e.g., HR4, HR23 and Mancellium) that contain more W and Al. Aluminum in the alloy leads to an increased resistance to carbon diffusion, but it also has a detrimental impact on strength. Cronite alloys like Mancellium, HRX23, HRX10 or HRX11 contain precisely balanced amounts of Al, ensuring perfect carburizing protection and high creep strength. Such alloys experience very small dimension changes over hundreds of cycles, and the base material remains nearly unaffected by diffused carbon beyond 0.1 mm from the surface.

    Production of these high-tech alloys requires highly qualified metallurgists using the latest melting technologies. Only the producers that manage the whole production of Al-containing superalloys will be able to supply parts for LPC.

    Use of high temperatures in LPC is the next challenge and milestone for fixture producers. Since increasing the working temperature by 100?C leads to a tenfold increase in the diffusion speed of carbon, pressure exists to increase the working temperature up to 1050?C (1922?F).

Quenching Cycles

Rapid cooling is an inseparable part of heat treatment. Processed parts undergo quenching just once, but fixtures must be quenched many times. In order to avoid cracking during heat-treat cycles, it is necessary to combine the proper alloy and a precise dilatation-free design. Such a combination allows the fixtures to endure multiple cycles. Even a small mistake in design could lead to premature cracks that can occur during multiple cycles. Some producers have increased section thickness to reduce crack development. Such fixtures work fine in terms of strength, but the increased weight results in a higher total power consumption. This commonly leads to a longer processing time and significantly slower quenching of the parts.

    A properly designed fixture using material with optimum strength values can do the same job with significantly lower weight. Proper design can result in saving approximately 50% of ballast weight. This makes it possible to decrease the processing time by hours, reduce the pressure of cooling gas and save money by not heating the ballast. Safe Cronite’s design testing equipment includes a large creep laboratory (Fig. 1) and thermal fatigue machine (Fig. 2) where the strength values for each alloy are measured. On a thermal fatigue machine, the test piece is stressed and exposed to repeated heating and quenching cycles. Such testing simulates the real working environment of the fixtures. For lifting devices, we use the results of high-temperature tensile tests (Fig. 3).

Heat-Treatment Fixtures

As was previously discussed, proper design of fixtures is very challenging. For LPC, Safe Cronite commonly provides dedicated fixtures based on the design of treated parts. A customer supplies the design of the parts to be treated and the parameters of the furnace, and we bring the solution. With a custom (dedicated) fixture design, the heat-treat payload is much larger. Figure 4 shows a typical Cronite fixture that was designed based on customer guidelines. It is a dilatation-free casting providing high strength together with long service life. The wall thickness of the fixture was calculated using FEA analysis.

Alloy Development

Aerospace alloys were the development model due to their high strength and carburizing resistance. Development was also inspired by the idea of applying a strong base metal enhanced by a special coating or surface treatment.

    New alloy requirements include:

•  Carburizing growth lower than 0.1% over 500 LPC working cycles

•  Creep-rupture strength 12 MPa/1000?C/10,000 hours or 8MPa/1050?C/10,000 hours

•  Good foundry properties

    The composition of selected alloys can be seen in Table 1. The exact amount of elements was tested within these chemistry ranges in order to achieve best properties.

    Alloys were stress-rupture tested in accordance to ISO 204 at Cronite’s accredited laboratory in the Czech Republic. The amount of Al showed very significant impact on the creep strength. Therefore, its amount was kept as low as possible in all alloys (Table 2/Fig. 5). All tested alloys achieved strength values comparable with the requirements. As a result, all of them were selected for prototype production (Table 3/Fig. 6).

Coating Development

The laboratory studied two types of coating. Spray coating creates a protective surface using CrSiAl oxides, and diffusion coating saturates elements into the surface. Much better results were achieved with diffusion coating, which is very resistant to quenching.

Carburizing Resistance

All alloys and surface treatments were tested in 500 LPC cycles (2,500 run hours). The testing cycle consisted of heating up to 950?C (1742?F) with a carburizing gas injection and diffusion and gas quenching. The dimensions and weight changes were measured every 50 cycles. All new alloys and diffusion coatings achieved very good results (Fig. 9). Compared with standard Cr-Ni austenitic steels, extreme differences were achieved. Over 500 LPC cycles, Cr-Ni steel parts changed dimensionally five times more than the new alloys. Such difference in practice rules out automatic loading because the position of parts will change every cycle.

    With a coating, it is possible to employ the alloy with the best strength-to-fatigue-resistance rate. When high mechanical resistance of the surface of parts is required, material containing Al in base structure can be proposed.

Conclusion

Based on the results of existing research, the R&D department decided to produce the prototype fixtures that are being tested in automotive production. The best solution will become a Cronite standard for years to come. The R&D department also worked on the construction of a special device for efficiently coating the fixtures. Safe Cronite will be able to support the new higher-temperature LPC process as soon as it enters the market. Some of the solutions are already being supplied to customers by special request. IH

For more information:  Contact Marco Möser, vice president, North American Cronite, 37162 Sugar Ridge Road, North Ridgeville, OH 44039; tel: 440-353-6594; fax: 440-353-6599; e-mail: marco.moser@safe-cronite.com