Stainless Steel

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Precipitation Hardening Stainless Steel which is chromium-nickel stainless, which contain alloying additions such as aluminium, copper or titanium that allow them to be hardened by a solution and that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they develop their high strength and hardness through a variety of heat treatments and they also have the corrosion resistance of austenitic steel. Precipitation Hardening stainless steels, or hardenable chromium-nickel alloys, are classified as martensitic, semi-austenitic or austenitic in the aged condition. The martensitic (such as Type 630) and semiaustenitic (such as Type 631) can provide higher strength than the austenitic (such as Type 660, also known as A286).

They are used in aircraft parts and commonly viewed as bar alloys, but are also available in flat roll products with a very high strength-to-weight ratio. The martensitic PH steels are used in aerospace, chemical and petrochemical, and food processing applications. Semi-austenitic grades are 17-7 PH and PH 15-7 Mo. They are austenitic in the annealed state, but martensitic in the hardened condition. Other grades of PH alloy stainless steels include 17-4 PH and 15-5 PH. The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630.
The PH grades achieve high tensile properties in heat treated conditions. Applications for PH alloy steels include aerospace components, flat springs and retaining rings, among others. The advantage of precipitation hardening steels is that they can be supplied in a "solution treated" condition, which is readily machinable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.

Characterization
Precipitation hardening steels are characterized into one of three groups based on their final microstructures after heat treatment. The three types are martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286).

Applications

• Gears
• Valves and other engine components
• High strength shafts
• Turbine blades
• Moulding dies
• Nuclear waste casks

Corrosion Resistance
Precipitation hardening stainless steels have moderate to good corrosion resistance in a range of environments. They have a better combination of strength and corrosion resistance than when compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to that found in grade 304 stainless steel. In warm chloride environments, 17-4 PH is susceptible to pitting and crevice corrosion. When aged at 550 DegreeC or higher, 17-4 PH is highly resistant to stress corrosion cracking. Better stress corrosion cracking resistance comes with higher ageing temperatures.
Corrosion resistance is low in the solution treated (annealed) condition and it should not be used before heat treatment.

Hot Working
Hot working of 17-4 PH should be performed at 950 Degree-1200 DegreeC. After hot working, full heat treatment is required. This involves annealing and cooling to room temperature or lower. Then the component needs to be precipitation hardened to achieve the required mechanical properties.

Weldability
Precipitation hardening steels can be readily welded using procedures similar to those used for the 300 series of stainless steels. Grade 17-4 PH can be successfully welded without preheating.

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Conquest Steel & Alloys is a supplier and stockholder of 253MA Stainless Steel is a lean austenitic heat resistant alloy with high strength and outstanding oxidation resistance. UNS 30815 and 253 MA maintains its heat resistant properties by advanced control of micro-alloy additions. 253MA contains a fairly low nickel content, which gives it some advantage in reducing sulphide atmospheres when compared to high nickel alloys. The inclusion of high chromium, silicon, nitrogen and cerium contents gives the steel good oxide stability, high elevated temperature (creep) strength and excellent resistance to sigma phase precipitation. The austenitic structure gives this grade excellent toughness, even down to cryogenic temperatures. The high mechanical strength of Conquest steel & alloys 253 MA allows for thinner tube wall constructions compared with using materials of the ASTM 310 type. Despite the material's high strength, ductility is not reduced, and good formability is maintained. A wide variety of components requiring high strength at elevated temperatures such as heat exchangers, kilns, stack dampers and oven components are common applications for 253 MA.

Main characteristics of 253 MA

• High creep- rupture strength
• Very good resistance to isothermal and, particularly, cyclic oxidation
• Very good resistance to combustion gases
• Good structural stability at high temperatures
• Good weldability
• Excellent oxidation resistance to 2000 DegreeF

Approx. Rs 126 / KgGet Latest Price

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Austenitic, or nonmagnetic stainless steels, are classified in the 200 and 300 series, with 16% to 30% chromium and 2% to 20% nickel for enhanced surface quality, formability and increased corrosion and wear resistance, and are nonhardenable by heat treating. These steels are the most popular grades of stainless produced due to their excellent formability and corrosion resistance. All austenitic steels are nonmagnetic in the annealed condition. (Depending on the composition, mainly the nickel content, austenitics do become slightly magnetic when cold worked.) Austenitic stainless steel grades include: Type 201, NITRONIC® 30, 301, 304, 305, 309S, 316, 316L, and 321. Austenitics are used for automotive trim, cookware, food and beverage equipment, processing equipment, and a variety of industrial applications.

Austenitic stainless steels are the most weldable of the stainlesses and can be divided rather loosely into three groups: common chromium-nickel (300 series), manganese-chromium-nickel-nitrogen (200 series) and specialty alloys. Austenitic is the most popular stainless steel group and is used for numerous industrial and consumer applications, such as in chemical plants, power plants, food processing and dairy equipment. Austenitic stainless steels have a face-centered cubic structure. Though generally very weldable, some grades can be prone to sensitization of the weld heat-affected zone and weld metal hot cracking. 

 

Super austenitic grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types.

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Martensitic, or hardenable stainless steels, are classified in the 400 series, usually with 11.5% chromium up to 18% chromium, with higher levels of carbon than ferritics, and are capable of being heat treated to a wide range of hardness and strength levels. Commercially produced AISI grades of this class are Type 410, 410H, 420, and 420HC. Martensitic stainless steels are used extensively in cutlery, sport knives and multi-purpose tools.

Martensitic stainless steels can be high or low-carbon steels built around the Type 410 composition of iron, 12% chromium, and up to 1.2% carbon. They are usually tempered and hardened. Tempered martensite gives steel good hardness and high toughness; used largely for medical tools (scalpels, razors and internal clamps).  Untempered martensite is low in toughness and therefore brittle.

Martensitic stainless steels work in the same way as many low alloy steels. Carbon is the key element. They have a structure similar to the ferritics with a 'body-centred tetragonal' (bct) crystal lattice. 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 in a somewhat distorted atomic matrix. This is known as body-centred tetragonal. The distortion of the atomic matrix leads to the hard martensitic structure. The higher the carbon level the harder is the martensite. In the as-quenched condition, martensitic steels are virtually useless as they have insufficient impact toughness.

Due to the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels. They are classed as a "hard" ferro-magnetic group. In the annealed condition, they have tensile yield strengths of about 275 MPa and so they are usually machined, cold formed, or cold worked in this condition. 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

Applications:

• Knife blades
• Cutlery
• Fasteners
• Surgical instruments
• Shafts
• Springs

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Ferritic stainless steels are plain chromium stainless steels with a chromium content varying between 10.5 and 18% and a low carbon content. Ferritic stainless steel are magnetic and cannot be hardened by heat treatment and are always used in the annealed condition. Ferritic, or nonhardenable stainless steels, are classified in the 400 series and are normally specified due to superior corrosion resistance and resistance to scaling at elevated temperatures. With inherent strength greater than carbon steels, ferritic provide an advantage in many applications where thinner materials and reduced weight are necessary, such as automotive emission control systems. They are nonhardenable by heat treating and are always magnetic. Commercially available AISI grades are Type 409, Aluminized 409, 410S, 430, 434, 436, 439, Aluminized 439 and 444.

Ferritic stainless steel consists of iron-chromium alloys with body-centered cubic crystal structures. They can have good ductility and formability, but high-temperature strengths are relatively poor when compared to austenitic grades. They are also not susceptible to stress corrosion cracking. Weldability is acceptable in thin sections but decreases as section thicknesses increase.
Some ferritic stainlesses (such as types 409 and 405) used, for example, in mufflers, exhaust systems, kitchen counters and sinks, cost less than other stainless steels. Other more highly alloyed steels low in C and N (such as types 444 and 261) are more costly, but are highly resistant to chlorides. Ferritic stainless steel include grades like 430 and contain only chromium as a major alloying element. They are known for their moderate corrosion resistance and poor fabrication properties. Fabrication properties can be improved by alloy modifications and are satisfactory in grades such as 434 and 444.

Application:

• Automotive exhausts systems
• Fuel lines
• Cooking utensils
• Appliances
• Furnaces
• Architectural trim
• Domestic appliances
• Petrochemical
• Heat exchangers

Types of Ferritic Stainless Steel:

• Group 1 (type 409/410L): These have the lowest chromium content of all stainless steels and are ideal for slightly corrosive environments where localized rust is acceptable. The least expensive of all stainless steels, type 409 was initially created for automotive exhaust systems silencers, but can now be found in automotive exhaust tubing and catalytic converter casings. Type 410L is often used for containers, buses and LCD monitor frames.
• Group 2 (type 430): The most commonly used ferritic steel, type 430 has a higher chromium content and is, consequently, more resistant to corrosion by nitric acids, sulfur gasses and many organic and food acids. In some applications, this grade can be used as a replacement for austenitic grade 304. Type 430 is often found in interior appliances, including washing machine drums, kitchen sinks, cutlery, indoor panels, dishwashers and other cooking utensils.
• Group 3 (type 430Ti, 439 and 441): Having better weldability and formability characteristics than Group 2 ferritic sheets of steel, Group 3 steel can be used to replace austenitic type 304 austenitic in a wider range of applications, including in sinks, exchange tubes, exhaust systems and welded parts of washing machines.
• Group 4 (type 434, 436, 444): With a higher molybdenum content, these ferritic stainless steel grades have enhanced corrosion resistance and are used in hot water tanks, solar water heaters, exhaust system parts, electric kettles, microwave oven elements, as well as the automotive trip. Grade 444, in particular, has a pitting resistance equivalent (PRE) to Grade 316, allowing it to be used in more corrosive outdoor environments