Nitrogen cooling for tool hardening

“Microstructure” and “crystal structure” are among the most frequently used words when metallurgists talk about steel. Besides the chemical composition of an alloy, the main factor in determining the properties of steel is the spatial arrangement of its atoms.

Austenite refers to an iron and carbon crystal structure which is found in many steels but is not always desirable. This structure only has a low degree of hardness and, as a rule, is difficult to cut, which considerably limits the processing possibilities. At Pilana, a Czech tool manufacturer, hardened steel is therefore frozen with nitrogen prior to machining, because at very low temperatures austenite is transformed into different, desirable structural forms. Messer has installed the technology for this and is supplying the nitrogen coolant. Among other things, Pilana produces wood milling machines and circular saw blades, as well as woodworking tools, plane blades and industrial cutters. The company employs 650 staff and is one of the largest tool manufacturers in Europe. The tools are manufactured in compliance with the DIN and ISO standards. A crucial factor here is, of course, the quality of the steel and this, in turn, is dependent on the crystal structure of the material. Depending on the tool and the application, the austenite in the steel needs to be transformed into a different crystal structure – martensite, which has an extremely high degree of hardness.

Lattice change
The classic method of transforming austenite into martensite involves repeated heating and cooling of the material. This takes a long time though and consumes a lot of energy. Cryogenic treatment offers a modern alternative, which saves both time and energy. In 2011, Pilana placed an order with Messer in the Czech Republic to install a facility for the cryogenic treatment of steel for cutting tools. This was designed to allow the steel to be cooled with cryogenic nitrogen to temperatures as low as minus 180 degrees Celsius and to be heated to a maximum temperature of plus 180 degrees Celsius. The centrepiece of the cryogenic process is the injection of liquid nitrogen into the refrigerated part of the chamber. Here, ventilators disperse the cryogenic gas so that its effect on the material is even. The entire cooling and reheating process is pre-programmed and centrally controlled to ensure accurate adherence to all the steel hardening parameters. This includes the cooling rate and time, the retention time for a predetermined temperature as well as the heating rate and time.

The advantage of controlled heating is that the entire process takes place in the box without air ingress. A further benefit is the rapid transition from the held temperature to ambient temperature, which saves a lot of time. Nitrogen and energy are also used very efficiently. Approximately two to three kilograms of liquid nitrogen is sufficient for one kilogram of material. A vacuuminsulated nitrogen supply pipe also helps to minimise the energy requirement.

Difference in quality
Besides being very efficient, the process also improves quality. When comparing the service life of cutting tool steels used in woodworking, there are measurable benefits from cryogenic treatment. The Austrian company Stora Enso Building and Living, based in Ybbs, has compared steel in tools (blades for removing tree bark) with and without subsequent cryogenic treatment at a minimum of minus 150 degrees Celsius. Cryogenic treatment reduces the tension in the material and facilitates the formation of particularly fine martensite needle grains – a prerequisite for a robust crystal structure. It therefore also facilitates improved cutting performance. Consequently, steels for tools that have been cryogenically treated have a significantly longer service life.