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Martensitic Stainless Steels

• satyendra
• April 28, 2014
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• AISI 400 series, bct, corrosion, heat treatment, Martensite, quenching, tempering,

Martensitic Stainless Steels

Stainless Steel can be defined as iron-based alloys which contain a minimum of about 12 % chromium (Cr), the quantity needed to ensure a high corrosion resistance by the formation of a passivating oxide film. This quantity of chromium needs to be dissolved within the metallic matrix in order to ensure passivation. While the iron-chromium (Fe–Cr) system forms the basis, modern stainless steels, besides chromium, also contain other alloying elements whose presence improves specific properties (i.e. hardness, tensile strength, ductility, and corrosion resistance etc., such as nickel (Ni), manganese (Mn), molybdenum (Mo), copper (Cu), titanium (Ti), silicon (Si), niobium (Ni), aluminum (Al), sulphur (S), and selenium (Se). The hardness of the martensitic stainless steels is mainly determined by the carbon content, whereas hardenability is because of other alloying elements.

Stainless steels are frequently classified as per their micro-structure into five types namely (i) austenitic, (ii) ferritic, (iii) martensitic, (iv) duplex (ferritic–austenitic), and (v) precipitation-hardenable (PH). More common types are austenitic, ferritic, and martensitic stainless steels, which have been discovered in the first decade of the 20th century. The different types of stainless steels possess different properties.

Martensitic stainless steels tend to be forgotten, perhaps since they are not made in large quantities compared with austenitic and ferritic grades. They are not as ‘exciting’ as some of the newer duplex grades. However, martensitic stainless steels play a big and frequently unseen role in the present-day world. However, these steels are important and have several applications.

Martensitic stainless steels are identified by AISI (American Iron Steel Institute) 400 series numbers. These contain chromium as the principal alloying element. In the annealed condition, these steels have basically a ferritic micro-structure and are magnetic. On heating beyond the critical temperature, the ferrite transforms into austenite. If then rapidly cooled to below the critical temperature, the austenite transforms into martensite. In several respects, the martensitic stainless steels are similar to the quenched and tempered carbon or alloy steels whose mechanical properties can be varied through heat treatment. Whether or not, the taking place of transformations depends upon content of the alloying element, especially the contents of chromium and carbon. Addition of other alloying can also affect the transformation.

Historical aspects – The characteristic body centered tetragonal (bct) martensitic micro-structure was first observed by German microscopist Adolf Martens around 1890. In 1906, L.B. Guillet published in France a study on the constitution and properties of ternary steels, containing chromium and nickel. A.M. Portevin, a Frenchman, published in 1909 in England, a study on martensitic and ferritic steels, containing 17.4 % chromium and 0.12 % carbon, which in other terms is AISI 430. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless-steel alloy. This patent was not granted until 1919.

In October 1912, F. Krupp (a German company) entered the German Patent Office, in Berlin, with the patent requests DRP 304126 and 304159: ‘Fabrication of objects that require high corrosion resistance ——-‘. Both patent requests did not mention the true inventors and they were registered in the name of the company’s patent bureaucrat called Pasel. From the initial work of Strauss and Maurer, two classes of stainless steels have been developed namely the martensitic VM steels (containing 0.15 % carbon, 14 % chromium, and 1.8 % nickel) and the austenitic VA steels (containing 0.25 % carbon, 20 % chromium, and 7 % nickel). ‘V’ stands for Versuch that in German means trial. The products manufactured from these steels have been exhibited in 1914 Fair of Malmoe in Sweden.

Krupp was the first firm to commercialize these stainless steels. By the end of the first semester of 1914, some 18 tons of the V2A (austenitic) steel have been supplied to Badisch Anilin-und Sodafabrik (BASF) in Ludwigshafen. Thomas Firth & Sons Ltd. of Sheffield produced about 50 tons of the martensitic type in 1914. In 1915, in the United States, Firth- Sterling near Pittsburgh started its production. Ferritic stainless steels usage and commercialization started later on, probably around 1920, when Brown Bayley’s Steel Works Ltd. of Sheffield made its first 5 to 6 tons of a 11.7 % chromium and 0.07 % carbon steel.

In England, Harry Brearley, in two articles, described his experiences with corrosion resistant alloys containing 12.8 % chromium and 0.24 % carbon. ‘When microscopic observations of these steels were being made, one of the first things noticeable was that reagents would not etch, or etched very slowly, those steels containing low carbon and high chromium’, (H. Brearley, Daily Independent of Sheffield, 2.2.1924). The martensitic stainless steels have been discovered. Brearley’s patent covered the 9 % to 16 % chromium and less than 0.7 % carbon alloys.

Martensitic grades of stainless steel have been developed in order to provide a group of stainless steels which are corrosion resistant and hardenable by heat treatment. Martensitic grades of stainless steels can also be made with nitrogen and nickel additions but with lower carbon levels than the traditional grades. These steels have improved toughness, weldability, and corrosion resistance.

Nitrogen (N2) is an element always present in conventional steelmaking processes and has been considered to be harmful for several years. But recently, nitrogen has been recognized as an element which improves not only the corrosion resistance of martensitic stainless steels, but also their yield strength, creep strength, and toughness. The high-nitrogen grades are being more popular as they possess higher strength, toughness, and pitting corrosion resistance. 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.

It is to be noted that corrosion resistance in martensitic stainless steels is considerably lower if compared to other stainless steels. These steels are inadequate for usage in more aggressive media. It is well known that the properties of these steels are strongly influenced by heat treatments. The quantity of carbides in the quenched micro-structures exerts an important influence on the properties of these materials, e.g., hardness, resistance to corrosion, and wear.

Martensitic stainless steels can 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. Their corrosion resistance can be described as moderate (i.e., their corrosion performance is poorer than other stainless steels of the same chromium and alloy content). They are ferro-magnetic, 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 can be welded with caution, but cracking can be a feature when matching filler metals are used.

AISI 410 grade is the basic grade of martensitic stainless steel. AISI 420 grade is high hardness grade. AISI 431 grade is high corrosion resistant and toughness grade. AISI 440A, 440B, and 440C have increasing hardness after heat treatment. AISI 416 grade is welding consumable grade. Fig 1 shows the family relationships for standard martensitic stainless steels.

Fig 1 Family relationships for standard martensitic stainless steels

Martensitic stainless steels grades can also be produced as flat products mainly for blades of different kinds. The martensitic creep-resisting stainless steels are normally of the 9 % – 12 % CrMoV type frequently with additions of other elements such as niobium and tungsten.

Martensitic stainless steels are essentially iron-chromium-carbon (Fe-Cr-C) alloys, containing chromium in the range of 11.5 % to 18 % and carbon in range of 0.1 % to 1.2 % (super-martensitic grades have carbon less than 0.015 %). Alloying elements like molybdenum, vanadium, niobium, and copper are added for the improvement of specific properties. Fig 2 shows projection of the ternary Fe-Cr-C diagram.

Fig 2 Influence of carbon, nitrogen, and nickel on gamma-loop

The high strength of martensitic stainless steel 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. The most common additions include molybdenum, vanadium, and niobium.  These additions lead to a complex precipitation sequence. A small quantity (up to 2 %) of nickel 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.

Basic metallurgy of martensitic stainless steels – Martensitic stainless steels work in the same way as several low alloy hardenable steels. Carbon is the key element. Normally, when steels are heated, they transform from ferrite to austenite. On slow cooling, the steel transforms back to ferrite. However, with fast cooling through quenching in water, oil, or sometimes even air, the carbon atoms become trapped and distort the normal body centred cubic, bcc, ferritic atomic matrix. This is known as body-centred tetragonal (bct) structure. The distortion of the atomic matrix leads to the hard martensitic structure. The higher is the carbon level, the higher is the distortion, and the harder is the resulting martensite.

Carbon levels can vary from less than 0.1 % to over 1 % in the martensitic stainless steels. In combination with other elements, this wide variation allows a wide range of properties to be developed for specific applications. The hardness of the martensitic stainless steels is mainly conferred by the carbon content, whereas hardenability is imparted by other alloying elements. In low alloy steels elements such as chromium, nickel, and molybdenum are used to improve the ‘hardenability’ of the steel. Hardenability is a measure of the maximum section size which transforms to martensite on cooling. This is related to the ‘ruling section’. In stainless steels, chromium is present in large quantities and so have excellent hardenability. Nickel and molybdenum are also used. Molybdenum has the added advantage of increasing the steel’s corrosion resistance.

In the as-quenched condition, martensitic steels are virtually useless since they have insufficient impact toughness. Occasionally, lower carbon martensitic steels can be used in the as-quenched condition for wear resistance. The most normal treatment following quenching is tempering. Tempering involves heating the steel to somewhere between 200 deg C and 700 deg. C. The temperature and length of time at temperature determines the final properties of the steel. Tempering imparts a useful combination of strength and toughness. Fig 3 shows how the tempering temperature affects the strength of the martensitic stainless steel.

Fig 3 Effect of tempering temperature on strength of martensitic stainless steel

For optimizing the 12 % chromium martensitic stainless steels, other alloying elements, such as molybdenum, vanadium, and niobium are added. The major role of alloying elements is to strengthen the secondary precipitation and to stabilize the formation of carbides. The addition of molybdenum and vanadium, which are ferrite stabilizers, improves the strength and resilience after tempering. Niobium, in contrast to the molybdenum and vanadium, combines with carbon to form niobium carbides which are stable at a conventional austenitization temperature (1,050 deg C). In addition to a stabilizer of the delta ferrite, this element hence decreases the quantity of available carbon in solid solution.

Other alloying elements frequently observed in martensitic stainless steels are nickel (around 0 % to 5 % nickel) and molybdenum. Nickel entitles the usage of lower carbon contents and, as a result, higher toughness and corrosion resistance can be achieved. For soft but tougher martensitic stainless steels containing lower carbon levels, nickel content can reach up to 5 %. These soft martensitic steels are frequently precipitation hardened. Molybdenum improves pitting corrosion resistance, in addition to improvement in toughness. Copper improves machinability.

New martensitic stainless steels with properties which suit specific applications have been developed with the aim to provide high mechanical properties by surface treatments and to improve corrosion resistance. Two of such martensitic stainless steels which have, hence, been developed are (i) precipitation hardening martensitic stainless steels, and (ii) martensitic stainless steels with 12 % chromium.

New martensitic stainless steels, such as the XD15NW steel (X40CrMoVN16-2, UNS S42025, a high hardness, corrosion and fatigue resistance martensitic stainless steel grade) have, however, been developed with the aim to replace AISI 440C for cryogenic aerospace bearings because of its improved tribological and fatigue properties. The CX13VD steel X12CrNiMoV12-3) is a carburizing stainless steel which is also part of the stainless steels with 12 % chromium and steels with the addition of nickel, molybdenum, and vanadium. It is used in the aerospace industry and industrial applications, such ball screws, blade propellers, and gears.

The steel type AISI 410 (12 % chromium, 0.1 % carbon) is one of the most popular materials within the martensitic stainless grade of steels and is used in a wide variety of applications in general engineering. In the hardened condition, its yield strength (YS) can reach around 1,300 MPa, however upon tempering at higher temperatures its strength decreases considerably. The yield strength attained by a martensitic stainless steel depends mainly on its carbon level and some compositions can reach a yield strength of the order of 1,900 MPa. The maximum chromium level is around 18 % and some compositions have up to 1 % molybdenum. However, both chromium and molybdenum shrink the austenitic field, which can be expanded by carbon and nickel. For example, a steel containing 17 % chromium and 0.2 % carbon needs 2 % nickel for avoiding delta-ferrite formation.

As chromium level is increased, carbon level has to increase also in order to stabilize austenite. For the martensitic steels, it is, of course, necessary to form austenite from which martensite is obtained on cooling. For example, for a complete austenitization, steels containing 13 % chromium need to have carbon content in excess of 0.15 % and to be heated to at least 950 deg C. Steels containing more chromium, say 17 %, need to have carbon content higher than 0.3 % and to be heated to at least 1,100 deg C. Both nitrogen and nickel expand the gamma-loop, as shown in Fig 2b and Fig 2c respectively.

Martensitic stainless steels can be sub-divided into three sub-groups namely (i) low-carbon steels for turbines, (ii) medium-carbon steels for cutlery, and (c) high-carbon wear-resistant steels. The micro-structure of each group also has characteristic namely (i) martensitic needle-like micro-structure, (ii) very fine martensitic micro-structure, and (c) ultra-fine martensitic micro-structure containing mainly carbides, respectively. Moreover, higher carbon steels, such as the AISI 440C, or nickel containing, such as the AISI 431, can present large quantities (more than 30 % in volume) of retained austenite after quenching. Sigma-phase precipitation is of minor importance in this class of steels. Depending on tempering temperature and chemical composition, especially the chromium / carbon ratio, several carbides can precipitate, such as the M2X, M3C, M7C3, M23C6, and MC types. Fig 4 shows micro-structures of AISI 410 and AISI 420 martensitic stainless steels.

Fig 4 Micro-structures of AISI 410 and AISI 420 martensitic stainless steels

Low-carbon martensitic stainless steels containing up to 12 % chromium are the most commonly used steel in this class. Carbon content is low since the main application of these steels is for structural purposes where high mechanical strength as well as weldability, workability, and toughness are needed. They are used in blades or other components in steam turbines, pump shafts, axels, and other components for the chemical, petro-chemical and oil industries, railway axles, components for the pulp and paper industry, and oven parts which operate under 400 deg C.

A typical representative of the medium-carbon martensitic stainless steels is the AISI 420, containing 13 % chromium and 0.35 % carbon. Increasing the carbon content in martensitic steels leads to higher hardness and mechanical resistance with a corresponding loss in toughness and weldability. Also, the higher austenitizing temperature leads to the possibility of M23C6 grain boundary precipitation (Fig 4b). These steels are employed in cutlery, surgical and dental instrumentation, axles, valves, pumps, steam turbine blades and other components, plastic molds, and in the glass industry.

The main representative of the higher carbon martensitic stainless steels is the AISI 440 series. The high-chromium levels make adequate their usage in marine atmospheres or in seawater. High-carbon content stabilizes austenite, increases hardness, mechanical resistance, adhesive and abrasive wear resistance. While adhesive wear resistance mainly needs higher hardness, abrasive wear also needs primary carbides. Normally, these steels are not welded. Some typical applications are roller bearings, valve needles, and spray nozzles.

Properties – Martensitic stainless steels having a minimum of 12 % of chromium have a good combination of mechanical properties, including good ductility and high mechanical strength, as well as corrosion resistance because of the chromium content. These steels are austenitic in a temperature range of 1,000 deg C to 1,050 deg C, and the martensite transformation can occur even with air-cooling, because of very good hardenability. However, in terms of corrosion resistance, which can be decreased by precipitation of chromium-rich carbides, air-cooling is to be avoided. In order to produce a good combination of strength, ductility, and toughness, these steels are normally tempered at temperatures between 100 deg C and 150 deg C.

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 MPa 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 1,900 MPa. 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 (Rockwell hardness ‘C’ scale).

Martensitic stainless steels can 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 can be welded with caution when matching filler metals are used but cracking can be a feature.

All martensitic stainless steels are ferro magnetic. Because of the stresses induced by the hardening transformation, these stainless steels show 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, hence, able to attract other ferro magnetic objects. Apart from possibility of causing handling problems, the parts are able to attract bits of iron or steel which, if not removed, impairs the corrosion resistance. It is hence practical 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 non-destructive testing using the magnetic particle inspection method, unlike austenitic stainless steels.

Optimum corrosion resistance is achieved in the heat-treated i.e., hardened and tempered condition. Martensitic stainless steels are less resistant to corrosion. Their corrosion resistance can 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 alloying elements content).

The effect of nitrogen on localized corrosion resistance of martensitic stainless steels shows that inter-granular corrosion takes place effectively in martensitic micro-structures exposed to sulphuric acid solutions, and that nitrogen additions up to 0.2 % allow improving resistance to this kind of localized attack.

Processing and heat treatment – Martensitic stainless steels are normally 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 normally in the range 925 deg C to 1,070 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 increases with austenitizing temperature up to a maximum and then decreases. 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 the 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 highest effect. This means that in the higher alloyed martensitic grades the micro-structure contains retained austenite because of 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 get useful engineering properties, martensitic stainless steels are normally tempered. The tempering temperature used has a large influence on the final properties of the steel.

Normally increasing tempering temperatures below around 400 deg C leads to a small decrease in the tensile strength and an increase in the 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 because of the secondary hardening peak, at around 450 deg C to 500 deg C.

In the temperature range around the secondary hardening peak there is normally a dip in the impact toughness curve. Above around 500 deg C, there is a rapid reduction in strength and hardness, and a corresponding increase in ductility and toughness. Tempering at temperatures above 780 deg C of the steel, results into partial austenitizing with the possibility of presence of untempered martensite after cooling to room temperature.

Prior to final hardening and tempering heat treatments, martensitic stainless steels are annealed in order to be machined and cold worked. For example, an AISI 410 is annealed in the temperature range of 750 deg C to 900 deg C for 2 hours to 4 hours and furnace or air cooled. In such a condition, hardness is around 160 HB (Brinell hardness), yield strength 300 MPa, tensile strength 500 MPa, elongation 20 %, with an area reduction 60 %.

Prior to their final usage, martensitic stainless steels are submitted to the same heat treatment sequence as that for carbon steels, i.e., they are austenitized, hardened by quenching, and tempered in order to improve ductility and toughness. At high temperatures, their stable structure is austenitic and at room temperature it is a stable mixture of ferrite and carbide. Tab 1 gives chemical composition, temperature range for heat treatment, and mechanical properties of some martensitic stainless steels. Since the stainless steels have a low thermal conductivity, depending on the cross-sectional size and complexity, pre-heating in the temperature range of 550 deg C to 800 deg C prior to final austenitizing temperature is desired. Air heating can cause decarburization.

The formation of a more stable (ferrite + carbides) micro-structure is very sluggish and the tendency towards martensite formation (high hardenability) is very high. Hence, the majority of martensitic stainless steels form martensite on air cooling, even for sections which are up to around 300 mm in thickness. Hardening media can be air or oil. While oil cooling is preferred in order to avoid carbide precipitation, air cooling can be needed to avoid distortions in more complex sections. Martempering is also possible in this class of steels.

Martensite hardness depends necessarily on carbon content varying from around 35 HRC for a 0.1 % carbon to 60 HRC for 0.5 % carbon, thereon increasing little for carbon higher than 0.5 %. For low-carbon martensitic steels, such as the AISI 410, the ‘Ms’ and ‘Mf’ temperatures are relatively high, 350 deg C and 250 deg C, respectively, and decrease with increasing carbon content. High-carbon steels can present retained austenite and a sub-zero treatment at around -75 deg C, immediately after hardening, is desired. Considerable progress has been made recently. Normally, martensitic stainless steels rarely achieve hardness higher than 60 HRC, even for high-carbon levels, because of the retained austenite. A careful combination of carbon and nitrogen contents, together with cold working and deep-freezing, can result in hardness between 61 HRC (in this case even after tempering) and 68 HRC.

Double tempering is also very common. Tempering temperature is determined by the needed mechanical properties. In general, temperature range of 420 deg C to 600 deg C is to be avoided, since within this range, brittleness can be induced apart from loss in the corrosion resistance.

Wear of martensitic stainless steels -In order to develop suitable martensitic stainless steels with better resistance to wear in a certain environment, it is important to know which factors are mainly influencing the wear rate and wear mechanisms. The most typical wear phenomena identified in treated and / or untreated martensitic stainless steels are ploughing (abrasive wear) and adhesive wear. In several cases, delamination and surface and subsurface cracks are also observed. Martensitic stainless steels show high surface hardness and the presence of carbides which promotes the generation of hard-wear debris particles and abrasive wear. However, heat treatments in martensitic stainless steels (quenching, and tempering) generate residual stress in the bulk material, which promotes the formation of cracks. Fig 5a shows a schematic illustration of the main wear mechanisms of martensitic stainless steels while rubbing in a dry contact.

Fig 5 Wear mechanism and formation of abrasive groove

Micro-structure plays an important role in the wear behaviour of martensitic stainless steels. The quantity of retained austenite, as well as the quantity of carbides, modifies wear behaviour. The schematic representation of the formation of the abrasive groove in a tool steel with retained austenite with a microstructure formed of carbide particles dispersed in a martensitic matrix is shown in Fig 5b. It is seen that the stress-induced transformation of the retained austenite (up to 15 %) into martensite and the consequent work hardening decreases the wear coefficient when increasing the applied load in the conditions as shown in Fig 5b(i) and Fig 5b(ii). When carbides are present in the micro-structure of the material, however, the extraction of these carbide particles, which increases with increasing load, is responsible for increasing the wear coefficient.

Large efforts have been made for the minimizing of wear in martensitic stainless steels. Results have shown that plasma nitride is an excellent candidate for improving the wear resistance of martensitic stainless steels for dry contacts.

Corrosion and passive behaviour of martensitic stainless steels – The corrosion resistance of martensitic stainless steels is normally lower than that of the other types of stainless steels. However, with at least 11.5 % chromium, they are genuine stainless steels giving a considerably improved corrosion resistance compared with low alloy steels. One has only to think of kitchen knives to realize that martensitic stainless steels are corrosion resistant in moderate conditions. The higher chromium grades such as grade AISI 431 offer corrosion resistance just below, or comparable to that of austenitic stainless-steel grade AISI 304 in several environments. No martensitic stainless steel can compare with austenitic stainless-steel grade AISI 316 or higher grades for corrosion resistance.

The outstanding corrosion resistance of stainless steels results from the presence of a thin oxide film on the metal surface. This oxide film is more enriched in chromium than the substrate, and typically 1 nano-meter (nm) to 3 nm thick. The alloy composition, the environment, and conditions of formation are some of the factors affecting the passivation phenomenon which happens to be very complex. Although the good mechanical properties and the corrosion resistance is still an issue. For this, several grades of martensitic stainless steels have been studied with different chemical composition and heat treatments. Majority of the studies are focused on the improvement of the properties of the martensitic stainless steels by surface treatments such as ion implantation, laser surface melting, or plasma nitriding. These surface treatments not only increased their surface hardness but also improved the corrosion resistance of these steels, when compared to the bulk alloy steels.

Martensitic stainless steels spontaneously passivate by forming an oxide layer mainly composed of iron and chromium and around 3 nm to 5 nm in thickness. The iron / chromium ratio varies from 1.5 to 7.2, depending on the studied conditions. Thickness and structure of the passive film of martensitic stainless steels are markedly influenced by the heat treatments applied to the steels. It has been found that the chromium-rich carbides precipitation reduces the passive film and pitting corrosion resistances. Micro-structures with high content of retained austenite prevent the formation of chromium precipitates and preserve higher chromium content in solid solution (leading to thicker anodic films), and the pitting potential linearly increases with the volume of retained austenite.

In presence of certain anions, in particular chloride, metals lose their passivity above a critical potential, called the pitting potential, and metal dissolution takes place from local sites where the passive film breaks. The localized dissolution leads to the formation of deep pits on a passive surface. Pitting corrosion needs the presence of aggressive anions, most frequently of chloride ions, and of an oxidizing agent such as oxygen or ferric ions. A corrosion cell forms between the growing pit which is the anode and the passive surface surrounding the pit which serves as the cathode. Since the anode / cathode surface ratio is small, the dissolution rate inside the pit can be quite fast. The pitting potential as a property of the metal environment system, depends on a number of factors such as the chemical composition and micro-structure of the metal, the surface condition and the presence of inclusions, the chemical composition of the electrolyte in particular the concentrations of the aggressive and non-aggressive anions, the temperature, and the prevailing convection conditions.

Pitting corrosion has been extensively studied. A correlation between sensitization and pitting potential has been established in the super-martensitic stainless-steel weldments, indicating that the probability of pitting corrosion is improved as the sensitization is increased. The critical conditions for the depassivation of the martensitic stainless steel are [Cl-] (chlorine ion) is higher than 0.4 M or pH is less than 1.5. When these conditions are reached, the stainless steel cannot re-passivate and the corrosion rate in the crevice increases. On the other hand, there is influence of the micro-structure on the corrosion behaviour of low-carbon martensitic stainless steel, concluding that pits normally initiate at sub-grain boundaries in martensite grains and at grain boundaries between martensite grains. Pits have been found to grow first iso-tropically, and then they propagate along preferential paths corresponding to sub-grains and grains with spread grain orientation values.

Pitting has been identified as the main corrosion mechanisms, while inter-granular and crevice corrosion have been also identified as secondary corrosion mechanisms. Inter-granular corrosion is a consequence of tempering associated to surface treatments.

Tribo-corrosion in martensitic stainless steels – Knowledge about the mechanisms of the simultaneous action of wear and corrosion leads to the development of new tools and components, which enable forecasting wear in sliding pairs of machines under regular operating conditions.

Tribo-corrosion is defined as a solid surface alteration which involves the joint action of relatively moving mechanical contact with chemical reaction in which the result can be different in effect than either process acting separately. Tribo-corrosion can occur under a variety of conditions (i.e. sliding, fretting, rolling, or impingement) in a corrosive medium. It involves different degradation phenomena depending on duty cycle location, i.e., corrosion, which occurs on the whole metal surface exposed to the corrosive fluids, wear accelerated corrosion that occurs in the wear track which has been de-passivated, and mechanical wear which occurs only in the contact area. In practice, it is important to be able to identify the contribution of corrosion and wear to material removal by tribo-corrosion in order to minimize material degradation and to prevent their early failure.

Plasma nitriding can effectively improve the surface hardness and the sliding wear resistance, as well as increase the corrosion–wear resistance because of the formation a thick surface layer. Abrasive wear is considered to be the main wear mechanism. Abrasive, and delamination wear have also been observed in the worn areas. These wear morphologies differ from the observations carried out under dry conditions. The chemical contribution to the overall degradation mechanisms modifies the worn patterns.

In a tribo-corrosion situation, in which the cyclic mechanical detachment of the passive film by the mechanical action is followed by a constant rebuilding of it, with re-passivation taking place during rubbing creates a dynamic evolution of the material in the wear track. Indeed, ‘fast re-passivation kinetics’ is a more important material behaviour than high hardness. If the friction coefficient is systematically lowered, the anodic dissolution rate is slower and the overall material behaviour is more stable at different potentials. Hence, not only mechanical but also electro-chemical, chemical, or material properties have to be considered to approach a tribo-corrosion situation.

Theoretical models are available nowadays and can be used and / or improved to understand tribo-corrosion of martensitic stainless steels. From this point of view, numerical tools to describe the dynamic evolution of wear and corrosion in a tribo-corrosion situation are very helpful to simulate different operating conditions and to get a deeper insight into those involved mechanisms. This understanding then establishes the basis for material and surface engineering which allows designing of metallic components for the specific tribo-corrosion situations.

Application of martensitic stainless steels – Martensitic stainless steels are specified when the application needs good tensile strength, creep and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance up to around 650 deg C. Because of their high strength in combination with some corrosion resistance, martensitic stainless steels are suitable for applications where the material is subjected to both corrosion and wear. Martensitic steels with high carbon content are frequently used for tool steels.

The combination of high strength, good toughness and moderate corrosion resistance allow martensitic stainless steels to be used in a wide variety of applications. Martensitic stainless steels are normally used for applications, where high mechanical performance is needed. These steels are extensively used for high-temperature and / or creep resistant applications. These steels are used in the power generation industry, aerospace industry, sporting equipment industry, for razor strip, for surgical instruments, and as bulk material in a variety of industrial applications, such as hot working moulds, dies, wires, screws, springs, gauge blocks, fasteners, blades and cutting tools, gears, shafts, propellers, pump impellers, ball bearings and races,  offshore oil and gas components, mixer and stirrers, and industrial knives etc., where high strength or wear resistance and moderate corrosion resistance are needed. They are also used in the petrochemical industry for steam and gas turbines blades and buckets. Typical other applications are aerospace, automotive, and hydroelectric engines, cutlery, defense, power hand tools, pump parts, valve seats, chisels, and bushings and etc.

Several of the above applications are hidden to the majority 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.