MATERIALS • PROCESSES • FINISHES
such as renewed surface oxidisation, which for some components means that cavities cannot be cleaned, or the heat treatment has to be performed in a protective atmosphere, which is expensive. Distortion is also an issue, so that machining allowances have to be increased. Hybrid Steel 55 could not offer an immediate answer. Tat was because in components such as suspension arms it would result in hardness being too high, combined with low toughness. Furthermore, following hot forging and cooling, the steel would be hard to machine. Tat prompted Ovako to develop a new grade – Hybrid Steel 50 – designed around a hardness of 50 HRC. Te main difference from the other two grades is a reduced level of vanadium.
Fig. 2. Hybrid Steel 50 mechanical properties (AR = as rolled, AG = ageing)
IDEAL SOLUTION FOR NEAR NET SHAPE FORGING Hybrid Steel 50 is ideal for near net shape forging as it does not need to be quenched to achieve a martensitic structure after hot forging. Tis avoids the need for an
environmentally harmful quenchant. It also minimises component distortion, enabling a reduction in the need for final machining processes. Te decrease in scrap and need for less handling increases productivity and saves machining tool costs. A particular advantage is that the new steel is straightforward to machine in a relatively soft condition after hot forging. Ten, low-temperature precipitation tempering, typically between 500-620°C, will provide the final hardness required for the component. Te benefit is that there is very low distortion and only minimal surface oxidisation. Significant cost savings can be possible, especially as only a simple ageing furnace is required. Tis production route could also help
minimise CO2 emissions for some components. Te final strength of Hybrid Steel
50 will vary considerably according to the specific process applied, as shown in Fig. 2. In general, it performs better than quenched and tempered steels. Te development of Hybrid Steel 50 is opening up new opportunities for cost- effective near net shape forging. It is just one of a number of options for new grades currently being explored for the innovative family that is primed for growth.
Patrik Ölund is head of group R&D at Ovako.
www.ovako.com
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