Stainless steel was invented in Sheffield, UK by metallurgist Harry Brearley during the early 20th Century. Brealey’s stainless steel contained around 0.2 weight % of carbon and 6 – 15 weight % of chromium. This alloy was originally developed in response to the problem of erosion/wear in gun barrels, but its durability, hardness, and stainless properties were found to be very desirable in other applications such as knives and cutlery. In the modern age, there are many different grades of stainless steel for a multitude of applications containing various amounts of Cr, Mn, Ni, and carbon.  The available grades of stainless steel can be classified into five basic families by their crystalline structure: ferritic, martensitic, austenitic, duplex and precipitation hardened. If you think of stainless steel now it is likely that the first thing you mention is an austenitic or ferritic metal, in grades 304 316 or 430 as these are the most commonly used. There are, however, other very useful grades such as the Martensitic Stainless Steel 1.4057 QT800 (ASTM 431 – SS2321). 1.4057 or Grade 431 is the ‘all-round’ engineering martensitic stainless steel, combining corrosion resistance with good strength and impact toughness.

1.4057 QT800 Stainless Steel

For martensitic stainless steel grades, the exact composition varies. But a typical 1.4057 stainless steel will contain: 15 – 17% chromium; 2 – 2.5% Nickel; 0.12 – 0.22% Carbon. It may also have small volumes of molybdenum, silicon, and phosphorous. In fact, Brearley’s first samples of stainless steel were martensitic. These alloys are magnetic and are generally formed in the annealed condition, then heat treated. Chromium is the primary alloying element of martensitic 1.4057 stainless steel, conveying moderate corrosion resistance to a material with inherently high hardness and strength. Typically, nickel concentrations of 2 – 2.5% are added as a stabilizing element to ensure a martensitic steel retains its toughness properties through heat treatment, enabling the fabrication of numerous component types. Martensitic stainless steels tend to be forgotten, perhaps because they are not in such demand compared to austenitic and ferritic grades. However, they often play a huge and often unseen part in modern infrastructure. The strength obtained by heat treatment depends on the carbon content of the alloy. 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 approaching 60 HRC. Optimum corrosion resistance is attained in the heat-treated, hardened and tempered condition. Other martensitic grades have been developed with nitrogen and nickel additions but with lower carbon levels than the traditional grades. These steels have improved toughness, weldability, and corrosion resistance.

Why Martensitic?

Martensitic stainless steels are similar to many low alloy steels where carbon is the key element. Typically, when steels are heated they transform from the ferrite to the austenite state. Upon slow-cooling, the steel reverts to ferrite. However, with fast cooling through quenching in water or oil, the carbon atoms become trapped in a somewhat distorted atomic matrix. This is known as body-centered tetragonal. The distortion of the atomic matrix leads to the hard-martensitic structure. The body-centered tetragonal martensite microstructure was first observed by Adolf Martens in 1890. The higher the carbon level the harder is the martensite. In the as-quenched and un-tempered condition, martensitic steels are virtually useless as they have insufficient impact toughness, they are brittle and unsuitable for engineering applications. Occasionally, lower carbon martensitic steels can be used in the as-quenched condition for wear resistance. The heat treatment condition of 1.4057 is QT 800 and it should be hot formed in the temperature range 1100-800°C. The most typical treatment following quenching is tempering (QT = quench-and-temper heat treatment). Tempering involves heating the steel to somewhere between 200 and 700°C. The temperature and length of time at temperature determines the final properties of the steel. Tempering imparts a combination of strength and resilience. Martensitic stainless steel can also be non-destructively tested using the magnetic particle inspection method, unlike austenitic stainless steel.

Properties and applications

By using 1.4057 QT 800 you get a high tensile strength stainless steel resistant to strongly oxidizing acids e.g., nitric acid and salt water with an exceptional blend of wear- and corrosion-resistance, impact toughness, and high strength. In addition, 1.4057 GT 800 is weldable and is generally easier to machine than ‘conventional’ austenitic steels. With an ultimate tensile strength (UTS) value upwards of 800MPa, 1.4057 is highly suitable for complex engineering projects in marine applications (shipbuilding and construction). This steel has good machinability and is widely used to fabricate drive and propeller shafts, bearings, spindles, pump and valve parts, piston rods, fittings, nuts and bolts. Other applications include: gears, hydraulic rams, valve stems, actuators, satellite parts, conveyor drive systems, hi-fi equipment stands, packaging machinery, crane pins, seat dampers for boats, mixing blades and golf clubs. The martensitic grade 1.4057 is also highly sought after for medical devices and medical tools (scalpels, razors and surgical clamps). This stainless steel has one of the best combinations of corrosion resistance, high strength and good impact toughness of all the stainless steels. In the annealed condition 1.4057 has a tensile yield strength of about 275 MPa and so is typically machined, cold formed, or cold worked in this condition before heat treatment to harden.

Military Applications

In the manufacture of military hardware such as small arm the majority of small parts for weapons are produced from stainless steel; the components obtained are then subjected to final heat treatments or to particular surface treatments. The martensitic stainless steels such as 1.4057 are ideal for use in this way for the production of firearms and their components. Components can be machined and then heat treated. Tempered martensitic steel such as 1.4057 gives steel good hardness and high toughness. Martensitic stainless has a particular for the fabrication of gun barrels here are two main advantages. First is a higher corrosion resistance when compared to an unlined, uncoated standard carbon steel barrel. However, a bare stainless steel barrel can still corrode because of the high duty cycle and needs to be looked after. due to the finishes applied to the inside and outside of the barrel. The second advantage to stainless steel vs. plain carbon steel as it relates to barrels is the ‘toughness’ of the metal. Stainless steel such as 1,4057 is more resistant to heat and abrasion than plain carbon steel. All else being equal, a stainless steel barrel is expected to experience less throat erosion than a plain carbon steel barrel, given identical firing schedules.

Summary

Martensitic stainless steels such as 1.4057 have a long history right back to Brearley’s discovery in 1912. Its combination of strength, toughness and moderate corrosion resistance makes it ideal for a wide range of applications. Although not used in large quantities compared to austenitic and ferritic grades, they are a crucial part of the stainless steel armory.

References

  1. EN 10088-2: Stainless steels – Part 2: Technical delivery conditions for sheet/plate and strip of corrosion resisting steels for general purposes
  2. http://www.worldstainless.org/Files/issf/non-image-files/PDF/TheStainlessSteelFamily.pdf
  3. Machining of Stainless Steels and Super Alloys: Traditional and Non-traditional Techniques, Helmi A. Youssef, 2016
  4. Advances in Stainless Steels, Baldev Raj et al. (editors), 2010
  5. Budynas, Richard G. and Nisbett, J. Keith (2008). Shigley’s Mechanical Engineering Design, Eight Edition. New York, NY: McGraw-Hill Higher Education. ISBN 978-0-07-312193-2.
  6. Akhavan Tabatabae, Behnam; et al. (2009). “Influence of Retained Austenite on the Mechanical Properties of Low Carbon Martensitic Stainless-Steel Castings”. ISIJ International. 51 (3): 471–475. doi:10.2355/isijinternational.51.471.
  7. Rodney Carlisle; Scientific American (2005-01-28). Scientific American Inventions and Discoveries: All the Milestones in Ingenuity – From the Discovery of Fire to the Invention of the Microwave Oven. John Wiley & Sons. p. 380. ISBN 978-0-471-66024-8.