Technical and economic Advantages When Applying SSH Process and RHP steels

Using RHP (Reduced Hardness Penetration) steel and the offered SSH (Solid-Surface Hardening) method made it possible to obtain a high-strength state in a steel part, to increase structural strength of this part as well as of the entire structure containing this part, to lower the mass of the finished product and to extend its operating life.

Replacing commercial alloyed steels, types 60S2 and 50HGSF, with non-alloyed steels allowed the Manufacturer to lower the raw material cost value by more than 50%.

The SSH technique in operation made it possible to effectively solve ecology problems accompanying the traditional hardening in oil. Using conventional water as a hardening media makes the SSH method be distinguished advantageously from other thermal treatment methods using mineral oils and polymer liquids as a hardening media.

As a result, a SSH spring blade acquires unique qualities. The sheet surface generates a hardened 0.1 – 0.2 mm deep layer featuring 250-280 kg/mm² (2500 – 2800 N/mm²) breaking strength and 55-58 HRC. The core features 120-150 kg/mm² (1200-1500 N/mm²) and HRC. Thus, the spring blade displays strength properties which are the same as those obtained during commercial thermal treatment, while the most loaded case-hardened layer on the steel features ultimate strength properties that can be obtained today. Besides, steel case-hardened layer features elastic compression stress of 50-70 kg/mm² (500-700 N/mm²) that adds to its fatigue strength. Spring blade's higher surface hardness (55-58 HRC) provides higher wearing quality compared to that of the commercial spring blades (40-45 HRC).

Commercial springs vs RHP steel springs competitive tests carried out as per the program simulating hard working conditions - off-the-highway driving – produced exceedingly optimistic results. For the same mass of the manufactured products, operational life of RHP steel springs up to fatigue breaking turned out to be 4 to 5 times longer than that of the commercial springs. Decreasing the RHP steel spring weight by 25% (120 kg for RHP steel and 160 kg for 50HGFA steel) makes new modification’s operational life 2 to 3 times longer.

Thus, using RHP steel springs makes it possible to obtain a substantial economy both for metallurgists (economy in alloying elements) and car manufacturers (lower spring mass and price), as well as in the vehicle operation (longer spring lifetime).

RHP steels and SSH hardening method can be used effectively not only for simple but also for few-blade springs (parabolic variable-focus springs); in this case all the above advantages of these products are maintained. SSH for spring blades 11-20 mm thick is carried out similar to the commercial method both for eddy-current and furnace heating.

At the end of 2003, OAO "Chusovskoy Metallurgichesky Zavod" (OAO "ChMZ") initiated production of the rear lower-weight spring blades for KamAZ vehicles. The result of this work was the following:

• implementation into production of RHP steels instead of alloyed steels of the 50HGFA and 60S2A types;
• SSH hardening method for all spring blades using water as a hardening liquid (instead of traditional hardening in oil with medium-temperature tempering);
• creation and startup of a semi-automatic hardening plant.

The above work made it possible to lower spring weight by 30%, extend its lifetime by a factor of 1.5 to 2, lower the steel doping level and primecost at the OAO "ChMZ" by over 20%.