TechArchive

This article was originally written in the period 1995-2000

David Milborn of British Steel Engineering Steels explains the latest developments in air-cooled forging steels

Air-cooled carbon and micro-alloyed steels are steels which instead of being heat treated after forging, are cooled in controlled conditions, either in still air or in an air-stream created by a fan. In Europe many forgers now have the equipment and know-how to enable them to carrying out cooling accurately and reliably.

British Steel Engineering Steels produces a range of air-cooled carbon and micro-alloyed steels offering tensile strengths from 700 to 1000 N/mm2. Carbon steels are used for lower tensile strength applications in the 750-850 N/mm2 range while micro-alloyed grades offer UTS levels up to 1000 N/mm2.

For the component manufacturer, the primary advantage of adopting an air-cooled grade rather than a conventional heat-treated version are the significant cost savings that can be achieved. The use of an air-cooled micro-alloyed grade (38MnSiVS5) for hub flanges has resulted in savings of (UK pounds)0.52 per part. However, there are further benefits in terms of the properties and performance of the finished component, including:

• reduced variation in mechanical properties
• uniformity of properties across the section
• reduced distortion.

In most cases improved machinability is also obtained. An air-cooled forging steel developed by British Steel for Rover’s re-launched MGF is also treated with calcium in order to improve machinability.

Engineering Steels has developed its own process for adding calcium during secondary steelmaking in a way which prevents this highly reactive element from being contaminated by contact with the atmosphere or with the slag lying on the surface of the molten steel. The calcium is injected into the ladle using a steel-sheathed wire to ensure that it reacts only with the oxides in the depths of the ladle.

The Air-Cooling Process

The enhanced properties of air-cooled steels can only be reliably achieved if cooling is carried out using conveyor equipment offering optimum control. The crucial criteria are the speed of the conveyor, the control of temperature (via fans or shrouds to accelerate or retard cooling) and the length of the conveyor which should be sufficient to ensure that the individually spaced components cool to less than 600degC before being removed. If the conveyor is to be used for a range of components, a variable speed and moveable fans/shrouds may be required.

The degree to which cooling rate can be controlled also depends upon the section size of the component. Still-air cooling is successful with smaller components, while fan cooling is often needed for larger section sizes. A fast cooling rate leads to the formation of a higher proportion of perlite in the steel which in turn yields higher strength and ductility. In micro-alloyed steels, accelerated cooling gives rise to finer precipitates and higher strengths. Very slow cooling rates produce softer components with a higher proportion of ferrite and a coarser perlite inter-lamellar spacing.

Properties and Applications of Air-cooled Steels

Air-cooled steels have a successful track record for certain automotive applications such as crankshafts and conrods. However, until recently their use was restricted because of concerns about their impact resistance; their inherently coarse grain structure meant that they could not be used for components subject to high impact loads.

Enhanced alloy design, coupled with a greater understanding of in-service conditions such as fatigue, has now resulted in the development of a new generation of forging steels which can be used in a wide range of components and applications.

Further applications for air-cooled grades are the subject of current research and development initiatives at British Steel. The company is currently working to develop new steels which will be used for the manufacture of fracture split conrods in which the large end cap is separated from the rod by fracture splitting. Fracture split conrods have been specified for a number of new passenger car engines, particularly in the US.

A collaborative project between Engineering Steels and British Steel Forgings is focusing on the need to develop steels which can be easily fracture split but which will also offer excellent machinability.

• British Steel Engineering Steels
• Tel: 0114 288 2361
• Fax: 0114 288 5033