04. Martensitic Stainless Steels
Martensite, the hardest structural component in steels, is responsible for the high hardness in many sharp edge tools. The formation of martensite is a remarkable phenomenon in materials science and smiths have known for at least 3000 years how to make use of it to produce implements such as knives, axes and ploughs. Therefore, martensite has played a crucial role in the development of our civilization.
However, the first martensitic stainless steels, invented by Brearly in Sheffield and Krupp Stahl in Germany, came into the market immediately before the First World War. Brearly’s invention brought fame to Sheffield by providing the basis of the well-known cutlery business.
The exceptional hardness of martensite derives from solid solution of carbon leading to a crystal structure which is body centered tetragonal, thus deviating from the cubic structure distinctive of austenite and ferrite (Fig 1). Austenite, which is a prerequisite for the formation of martensite, is quite soft in its annealed condition. Owing to carbon dissolved in austenite and a subsequent phase transformation martensite can be formed by a quenching procedure whereby carbon is trapped in the position indicated in Figure 1. The crystal asymmetry this causes is the secret behind the extreme hardness of martensite. While high carbon martensite can reach a hardness of 1000 HV the hardness of austenite is typically one order of magnitude lower.
A subset of all martensitic stainless steels containing 9-12% chromium is used in coal fired boilers. Owing to recent research these have been modified to withstand temperatures as high as 600°C or even above. This development has led to higher work efficiency and less emissions of carbon dioxide per unit energy produced.
However, the emphasis of this column is on martensitic stainless steels with about 13% chromium used in consumer products. Examples are kitchen utensils and various other consumer products such as razors and shaving blades, which are more well-known to people in general. A selection of martensitic stainless steels is given in Table 1 below. Most of these fall within the specifications of UNS S 42026 (EN 1.4021) or are modifications based on this theme. One reason for modifying standard 42026 alloys is to improve the resistance to corrosion in applications where the standard composition is inadequate. Because pitting corrosion in chloride ion containing media (e. g. the kitchen) is a potential problem, it is quite helpful to increase the amount of chromium, molybdenum and even nitrogen slightly. For instance, cleaning of kitchen knives in dish-washers requires more than 13% chromium for corrosion prevention (Figure 2).
In some applications, wear resistance is an essential property. Sandvik 19C27 has been developed to meet this need in very demanding industrial applications where knives are used to cut synthetic fibre, paper and polymeric films. Because of the high carbon content, 0.95%, the volume fraction of carbides is quite high, a factor contributing to improved wear resistance. However, another effect of the high carbon concentration is that the stability range of carbides is expanded, leading to difficulties in avoiding the primary carbides during production. Since these are quite large, they cannot be dissolved during austenitizing and inevitably will be present also in the final product. Such a material is not suitable in applications where the edge quality is of prime importance, e.g. razor blades. The improved wear resistance of Sandvik 19C26 is, therefore, obtained at the expense of the quality of the edge.
Martensitic stainless steels optimized for razor blade applications are balanced to give an extremely fine dispersion of carbides in the delivery condition (Figure 3). This facilitates the dissolution of carbides during heat treatment and liberates enough carbon to make the martensite sufficiently hard. Carbide particles larger than 2 µm are unacceptable because of the risk of causing damage to the skin.
Table 1. A selection of stainless martensitic steels.
|UNS S 42026||0.32||0.2||0.3||13.5||-||Shaving heads, kitchen tools (butter knives, potato peelers, slicers)|
|UNS S 42026||0.38||0.4||0.6||13.5||1.0 Mo||Flapper valves, surgical instruments, cutters in electric shavers, meat saws, doctor blades|
|UNS S 42026||0.52||0.4||0.6||14.5||-||High quality edge tools requiring dish washing (kitchen knives, scissors)|
|UNS S 42026||0.6||0.4||0.4||13.5||-||Hunting and fishing knives, pocket knives, skate blades|
|UNS S 42026||0.62||0.2||0.6||14.0||0.11N||With enhanced corrosion resistance. Hunting and fishing knives, pocket knives, skate blades|
|UNS S 42026||0.68||0.4||0.7||13.0||Razor blades, scalpels and industrial knives|
|Sandvik 19C27||0.95||0.4||0.7||13.5||Wear resistant industrial knives for cutting synthetic fibre, paper and plastic films|
As we have seen, traditional martensitic stainless steels form martensite during rapid cooling. There are other ways of forming martensite. One is to introduce plastic deformation of an austenitic stainless steel. You are welcome to read about this in my next column about metastable austenitic stainless steels.
This article was first published in Stainless Steel World Magazine in June 2017.