Stainless Steel – a Special Material
Stainless steels are materials with outstanding properties. They are corrosion-resistant, temperature-resistant, conductive, weldable, can be easily formed and finished in a visually appealing way. All stainless steels have a minimum content of dissolved chromium of 12%, which is also referred to as chromium steel. Thanks to the dissolved chromium, the steel can form a protective chromium oxide layer in combination with oxygen. This layer with a thickness of only a few nanometres protects the steel against corrosion very effectively. An additional alloying of the steel with sufficient nickel leads to the formation of an austenitic matrix. Austenitic chromium-nickel steel stands out by its increased corrosion resistance compared to chromium steel. Colloquial names include V2A and V4A for instance. Austenitic steel is frequently used for domestic sanitary and kitchen equipment. To further support corrosion resistance, nitrogen is also added to the stainless steel, apart from chromium, nickel and molybdenum, and the content of carbon is possibly minimized. A representative of this group of materials is the 1.4429 (X2CrNiMoN17-13-3 / AISI 316LN) with very good corrosion resistance and low magnetizability.
Can Stainless Steel Be Hardened?
Although stainless steel involves a lot of positive properties, especially wear protection is not sufficient in many applications. Stainless austenitic steel and duplex steel are relatively soft. Classical methods such as nitriding increase wear protection, but destroy the good corrosion properties of the material at the same time. Hard coatings (chrome-plating, nickel-plating, PVD coating) are unsuitable due to the process engineering expenses and the “eggshell effect” (hard overlay coating – soft core). With regard to industrial production, apart from the quality of the product, the cost-effectiveness of the processes used is often crucial. The process chain of the coating and production to final dimension is more expensive than that of a thermochemical surface layer treatment with precise contour uniformity.
Properties and Areas of Application of Hardened Stainless Steel
Hardened stainless steel used in fields of application, where high demands are placed on corrosion resistance and wear protection. BORINOX® produces a surface which is up to 5 times harder and moves the operating limits of the previously soft steel. In particular, the improvement of abrasion resistance, of cavitation resistance, increased fatigue strength and the prevention of cold welding tendencies are outstanding features with respect to components treated with BORINOX®. The areas of application are varied and comprise the consumer goods and household goods sector, mechanical engineering and construction, medical technology, automotive and components industry.
Process for the Hardening of Stainless Steel
Basically, four different processes for the hardening of stainless steel can be taken into consideration:
- Thermal processes: defined heating up, holding and quenching leads to the formation of martensite in case of curable martensitic or precipitation-hadenable stainless steels. In this method, no further elements are added to the steel. In practice, these processes are known for instance from the hardening of knife blades.
- Thermochemical diffusion processes: By diffusing elements such as nitrogen, chromium, carbon and boron into the surface of a material, intermetallic compounds and new phases are formed in the peripheral area. The compounds and phases formed are subject to compressive stress and lead to the desired hardening effect.
- Coatings: When coating, a wear protection layer is applied to the steel. These include for example: chrome-plating, nickel-plating, thermal spraying and PVD coatings.
Work hardening: increase in hardness by means of massive deformation of the basic structure, which can be caused by means of punching processes and shot peening for instance.
Drawbacks of Classical Processes for the Hardening of Stainless Steel
Classically used solutions for the hardening of stainless steel have various disadvantages. Thus, the thermos-chemical nitriding process, for example, helps protect against mechanical wear, but at the same time the corrosion resistance of steel is destroyed due to the precipitation of chromium nitrides. The conservation of corrosion resistance, however, is of fundamental importance in many fields of applications. Martensitic hardening is only possible for steels with increased carbon content and ferritic/martensitic matrix. Austenitic steels and duplex steels are not eligible for thermal processes. Nevertheless, they represent the largest share of the stainless steels used. Coatings are particularly susceptible to spot loads and dissolve quickly when damaged. The amount of rework of complex geometries is very costly. Work hardening processes always lead to a deterioration of corrosion properties, since the hardness increase is due to the generation of lattice defects. Lattice defects in the edge region promote corrosion attacks.
The BORINOX® Process
The BORINOX® treatment produces a very hard and wear-resistant peripheral zone in the surface of stainless steel and special alloys. BORINOX® protects against abrasion, cavitation and fatigue, without deteriorating the corrosion properties of the steel. Furthermore, BORINOX® helps to improve the friction coefficient and effectively prevents cold welding of metallic combinations of materials. The BORINOX@ process is an environmentally friendly gas-based diffusion process in the low-pressure range. An innovative process oversaturates the periphery of the steels with carbon and/or nitrogen. The forced solution in the metal lattice leads to a surface hardness being higher up to 5 times without undesired precipitations of chromium compounds. The BORINOX® process is extremely dimensionally stable and is also particularly suitable for critical stamped parts (cutting edges, fatigue fracture zones). A unique distinguishing feature is also the possibility of partial treatment of surfaces.
Materials for the BORINOX® Process
The Borinox process is suitable for most stainless steels. These include austenitic steels (AISI 304, 1.4301, AISI 316(L), 1.4404, 1.4571…), duplex steels (1.4462, AISI 318LN,…), precipitation hardening steels (1.4542, PH 17-4, 1.4545, PH 15-5…), martensite (1.4057, 1.4122…) and ferrite (1.4005, 1.4105…). For duplex steels, which consist of austenitic and ferritic structural components, the treatment result with BORINOX® primarily stands out due to the uniform formation of the diffusion zone. Special alloys such as nickel-base alloys (Inconel® Haynes®, Hastelloy®…) and cobalt chromium alloys are also suitable for BORINOX® treatments. As a specialty of BORINOX®, the uniform treatment result should be mentioned in connection with an excellent corrosion resistance with strongly deformed stamped parts.