Martensitic Stainless Steels
Martensitic Stainless Steels
Martensitic grades of stainless steel were developed in order to provide a group of stainless steels which are corrosion resistant and hardenable by heat treatment. Martensitic stainless steels are essentially Fe-Cr-C alloys and are similar to carbon or low alloy steels with a structure similar to the ferritic steels. However, due the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels. The main alloying elements are chromium (10.5 % to 18 %), molybdenum (0.2 % to 1 %), no nickel (except for two grades), and carbon (0.1 % to 1.2 %). Major grades in the family of martensitic group of stainless steels are given in Fig 1.
Fig 1 Major grades of martensitic stainless steels
The characteristic body centered tetragonal martensitic microstructure was first observed by German microscopist Adolf Martens around 1890. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless steel alloy. This patent was not granted until 1919.
Also in 1912, Harry Brearley of the Brown – Firth research laboratory in Sheffield, England, while seeking a corrosion resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times. Brearley applied for a US patent during 1915.
The structures of martensitic stainless steels are body centered tetragonal (bct) and they are classified as a hard ferro magnetic group. In the annealed condition, these steels have tensile yield strengths of around 275 N/sq mm and hence they can be machined, cold formed, or cold worked in this condition. These stainless steels have good ductility and toughness properties, which decrease as strength increases. Martensitic stainless steels can be moderately hardened by cold working. These stainless steels are typically heat treated by both hardening and tempering to yield strength levels up to 1900 N/sq mm. The strength obtained by heat treatment depends on the carbon content of the steels. Increasing the carbon content increases the strength and hardness potential but decreases ductility and toughness. The higher carbon grades are capable of being heat treated to a hardness of 60 HRC.
Martensitic stainless steels may be heat treated, in a similar manner to conventional steels, to provide a range of mechanical properties, but offer higher hardenability and have different heat treatment temperatures. They are subject to an impact transition at low temperatures and possess poor formability. Their thermal expansion and other thermal properties are similar to conventional steels. They may be welded with caution when matching filler metals are used but cracking can be a feature.
All martensitic stainless steels are ferro magnetic. Due to the stresses induced by the hardening transformation, these stainless steels exhibit permanent magnetic properties if magnetized in the hardened condition. For a given grade, the coercive force tends to increase with increasing hardness, rendering these stainless steels more difficult to demagnetize. These stainless steels are not used as permanent magnets to any significant extent.
Cold working increases the coercive force of these steels changing their behaviour from that of a soft magnet to a weak permanent magnet. If parts of cold worked martensitic stainless steel are exposed to a strong magnetic field, the parts can be permanently magnetized and, therefore, able to attract other ferro magnetic objects. Apart from possibly causing handling problems, the parts would be able to attract bits of iron or steel which will, if not removed, impair corrosion resistance. It is therefore prudent to either electrically or thermally demagnetize such parts if they have been subjected to a strong magnetic field during fabrication.
Martensitic stainless steels can be tested by nondestructive testing using the magnetic particle inspection method, unlike austenitic stainless steels.
Optimum corrosion resistance is attained in the heat-treated i.e. hardened and tempered condition. Martensitic stainless steels are less resistant to corrosion. Their corrosion resistance may be described as moderate (i.e. their corrosion performance is poorer in comparison with the austenitic and ferritic grades of stainless steels of the same chromium and alloy content).
The effect of nitrogen on localized corrosion resistance of martensitic stainless steels showed that intergranular corrosion effectively takes place in martensitic microstructures exposed to sulphuric acid solutions, and that nitrogen additions up to 0.2 % [weight %] allow improving resistance to this kind of localized attack.
Martensitic grades of stainless steels can be developed with nitrogen and nickel additions but with lower carbon levels than the traditional grades. These steels have improved toughness, weldability and corrosion resistance.
Martensitic stainless steels are usually used in the hardened and tempered condition. The hardening treatment consists of heating to a high temperature in order to produce an austenitic structure with carbon in solid solution followed by quenching. The austenitizing temperature is generally in the range 925 deg C to 1070 deg C. The effect of austenitizing temperature and time on hardness and strength varies with the composition of the steel, especially the carbon content.
In general the hardness will increase with austenitizing temperature up to a maximum and then decrease. The effect of increased time at the austenitizing temperature normally indicates that there is a slow reduction in hardness with increased time.
Quenching, after austenitizing, is done in air, oil or water depending on steel grade. On cooling below the Ms temperature (starting temperature for the martensite transformation) the austenite transforms to martensite. The Ms temperature lies in the range 300 deg C to 700 deg C and the transformation is finished at around 150 deg C to 200 deg C below the Ms temperature.
Almost all alloying elements lower the Ms temperature with carbon having the greatest effect. This means that in the higher alloyed martensitic grades the microstructure contains retained austenite due to the low temperature (below ambient) needed to finish the transformation of the austenite to martensite.
In the hardened condition the strength and hardness are high but the ductility and toughness are low. In order to obtain useful engineering properties, martensitic stainless steels are normally tempered. The tempering temperature used has a large influence on the final properties of the steel.
Usually increasing tempering temperatures below about 400 deg C leads to a small decrease in tensile strength and an increase in reduction of area, while hardness, elongation and yield strength are more or less unaffected. Above this temperature there is more or less pronounced increase in yield strength, tensile strength and hardness due to the secondary hardening peak, around 450 deg C to 500 deg C.
In the temperature range around the secondary hardening peak there is generally a dip in the impact toughness curve. Above about 500 deg C there is a rapid reduction in strength and hardness, and a corresponding increase in ductility and toughness. Tempering at temperatures above the 780 deg C for the steel, results into partial austenitizing with the possibility of presence of untempered martensite after cooling to room temperature.
Developments in martensitic stainless steels
Martensitic stainless steels are also produced with low carbon content (0.06 % max) and with 3 % to 6 % nickel. These steels, called ‘martensitic-austenitic’ or ‘nickel-martensitic’, have a balanced composition that promotes stable austenite after hardening and tempering. They have relatively good weldability.
The low carbon types have been further developed into ‘super martensitic’ stainless steels. These stainless steels typically contain 11 % to 13.% of chromium, 2 % to 6.% of nickel, 0 % to 3.% of molybdenum and a maximum of 0.030.% of carbon and nitrogen. Their high strength is combined with good impact strength and weldability.
In addition to the standard grades, a large number of alloyed martensitic stainless steels have been developed for moderately high temperature applications. Most common additions include Mo, V and Nb. These lead to a complex precipitation sequence. A small amount (up to 2 weight %) of Ni is added which improves the toughness. The 12Cr-Mo-V-Nb steels are used in the power generation industry, for steam turbine blades operating at temperatures around 600 deg C.
Some corrosion-erosion experiments performed with martensitic stainless steels have shown that corrosion-erosion resistance of the high-nitrogen stainless steels is higher than that of the conventional stainless steel for the testing temperatures, in the range from 0 deg C to 70 deg C. This can be associated to the beneficial effect of nitrogen in solid solution in martensite.
Applications of Martensitic Stainless Steels
Martensitic stainless steels are specified when the application requires good tensile strength, creep, and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance upto approximately 650 deg C. Due to their high strength in combination with some corrosion resistance, martensitic steels are suitable for applications where the material is subjected to both corrosion and wear. Martensitic steels with high carbon content are often used for tool steels.
Martensitic stainless steels are used for surgical and dental instruments, wire, screws, springs, razor strips, blades and cutting tools, fasteners, gears and ball bearings and races, gauge blocks, moulds and dies etc. They are also used in the petrochemical industry for steam and gas turbines blades and buckets. Typical other applications are aerospace, automotive, hydroelectric engines, cutlery, defense, power hand tools, pump parts, valve seats, chisels, bushings, shafts, and sporting equipment industry etc.
Many of these applications are hidden to most of the public which probably explains why martensitic stainless steels do not have a prominent public profile. It is good to remind the general public that much of the modern world rests on martensitic stainless steels which is doing their job behind the scenes.