HERSTA – The specialist of high-quality steel

The special chemical composition of nitriding steels guarantees optimum nitriding treatment following mechanical processing. Introducing nitrogen to the surface of the material makes it possible to create surface hardnesses of more than 1,000 HV. Resistance to wear and corrosion increase accordingly, while the core remains ductile and there is little material distortion. This is achieved by optimizing the composition of the alloying elements molybdenum, chromium, aluminium and/or vanadium. Nitrided steels are typically used for cylinders, piston rods and worm screws in the plastics industry as well as spindles in machine tools.

Case hardened steels are case hardened following mechanical processing, in other words, carbon is diffused into the surface of the material in a process known as carburization. This produces a high surface hardness while the material retains a tough core. The achievable toughness and ductility of the core (in particular at low temperatures) depends on the composition of the alloy. As a rule, to guarantee good mechanical machinability case hardened steels are subject to an annealing or a prehardening and tempering process at the production site. They are used for components subject to the high levels of wear and high impact loads to which gears and bolts are typically exposed. Case hardened steels are not suitable for ambient temperatures above 200 °C because their surface hardness begins to decrease.

The carbon content of quenched and tempered steels is mostly between 0.2% and 0.6%. Quenched and tempered steels are chosen according to the balance of strength and ductility properties (based on the cross-section of the component) required for the respective application. Relevant factors are the chemical composition of the material, its thermal treatment and the operating temperature of the finished component. Quenched and tempered steels are produced either soft annealed or hardened and tempered at the production site. When in a prehardened condition, further heat treatment is not often required following mechanical machining. Quenched and tempered steels are used for crankshafts, axes, shafts, connecting rods, bolts and screws.

ZF steels are steels that have been specially standardized and patented by ZF Friedrichshafen AG. They are normally based on general standardized steels whose properties have been adapted to satisfy the needs of ZF Friedrichshafen AG. Amongst other aspects, these adaptations concern the chemical composition, thermal treatment, degree of straining and ultrasonic testing. The precise specifications depend on the respective application and are based on numerous ZF standards. For instance, there are different specifications for marine drive technology and industrial gear trains. Only steel works audited by ZF Friedrichshafen AG are allowed to produce case hardened steels to ZF Friedrichshafen AG standards.

Through-hardened ball and roller bearing steels have a carbon content of approximately 1%. Annealing components after mechanical processing makes it possible to achieve a surface hardness up to 60 HRC. To guarantee good mechanical machinability despite a comparatively high carbon content, ball and roller bearing steels are subjected to a special annealing process at the production site. This “annealing to speroidal cementite” is known as AC or spheroidizing annealing. Due to their high resistance to wear, ball and roller bearing steels are used in the manufacture of ball bearings, roller bearings, anti-friction bearings, worm gears, linear guides and ball screws. That makes it absolutely essential that the raw material has a high degree of purity.

High temperature quenched and tempered steels are used in applications subject to high temperatures. Special chemical analyses and high tempering temperatures when annealing the steel guarantee its strength will not decrease later when subject to high temperatures in the intended application or over longer periods of time. Some of the high temperature quenched and tempered steels we stock can be used in temperatures up to 530 °C. High temperature quenched and tempered steels are typically used in the construction of power stations and pressure vessels. As a consequence, stringent demands must be met to ensure compliance with specifications of the AD-Regelwerke (German ‘working group guidelines’ covering pressure vessels) and the European Pressure Equipment Directive.