Transactions of The Indian Institute of Metals����� ��� ��� �� ���� ��������� � ����� ���� �������

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High performance stainless steels for critical engineeringapplications

M. Narayana Rao

Mishra Dhatu Nigam Limited, P.O.Kanchanbagh, Hyderabad-500 058

Email: cmd.midhani@ap.nic.in

Received 29 July 2008Revised 19 November 2009Accepted 19 November 2009Online at www.springerlink.com© 2010 TIIM, India

AbstractMIDHANI has been producing special stainless steels for different sectors. Production of these steels has posed challenges with respect tocontrol over chemical composition, designing heat treatment parameters to meet the desired properties. The most challenging grades havebeen SS304L, 13-8 PH and 9Cr1Mo steels to name a few. The melting equipments were selected with utmost care and processing was doneto meet the specified properties. In case of 304L grade in order to meet corrosion resistance requirements, elements like Silicon, Carbon andSulphur were required to be controlled in very low limits. Production of 13-8 PH steels demanded that a combination of high strength andtoughness are achieved.

This paper highlights MIDHANI’s experience in producing these alloys and other special stainless steels.

1. Introduction

Stainless Steels are a family of versatile materials that hasbeen put into a wide variety of application by mankind.Stainless steels are iron-based alloys containing minimum12% chromium and upto 25% nickel with minor additions ofcarbon, nitrogen, molybdenum, tungsten, titanium, niobium,copper and selenium. It has a wide range of applications fromsmall pins to the construction of automobiles, petrochemical,space, aeronautical, ship building industries and nuclearpower stations. Certain grades of stainless steels, because oftheir biocompatibility are used for manufacture of biomedicalimplants. In fact steel touches every sphere of our daily life.

By and large stainless steel family consists of hundredsof grades with varieties of compositions and a large spectrumof mechanical properties. The corrosion and oxidationresistance of stainless steels have been significantlyimproved through fine-tuned chemical compositions andmicrostructural constituents, leading to the evolution of superstainless steels.

Over the years, MIDHANI has catered to the requirementsof Indian Space, Nuclear, Aeronautical and Defence sectorfor many high performance materials. A wide range of specialalloys – many of them being tailor made to customer’s specificneeds have been developed and supplied. The materialssupplied prominently include special alloy and stainlesssteels, superalloys and non-ferrous metals and alloys oftitanium and molybdenum. This has been possible with thehelp of state of the art facility and excellent quality assurancesystem available in Midhani.

2. Manufacturing facilities at MIDHANI

Midhani’s flexible manufacturing system permitscommercial scale production of wide spectrum of high

Keywords:special stainless steels; nitrogen to aluminum ratio; strength;corrosion resistance; toughness

technology alloys in multiplicity of mill forms conforming towidely varying specifications of customers. MIDHANIproduces a number of Speciality steels, Stainless steels,Maraging steels and Titanium and its alloys and Nickel,Cobalt and Iron based superalloys. Appendix 1 givesMIDHANI’S Stainless Steels and their equivalents. Primarymelting of ferrous alloys is carried out in Electric Arc Furnace,Air Induction Furnace, Vacuum Induction Refining Furnaceor Vacuum Induction Melting Furnace depending upon thespecification requirements. Secondary melting is carried outin either Vacuum Arc Remelting (VAR) or Electro SlagRemelting (ESR). However, titanium and its alloys aremanufactured by compacting the sponge and alloy additions.The compacts are kept on the former and welded to form asingle unit called electrodes, which are then remelted inVacuum Arc Remelting Furnace. Higher purity titanium alloysare manufactured by repeated remelting in Vacuum ArcRemelting Furnace. These are then known as double or triplemelted grades. Melting of Superalloy is usually done invacuum induction melting furnace to ensure close controlover chemical composition and cleanliness. The output ofthe Vacuum Induction Melting Furnace is subsequentlyremelted in Vacuum Arc Refining Furnace or Electro SlagRemelting Furnace. The ingots are processed to productsranging from bars, flats, rings, and rods to wires, tubes andfoil. Molybdenum and its alloys are produced throughpowder metallurgy route.

3. Speciality stainless steel

Since its inception, MIDHANI has been actively involvedin development and supply of a number of special stainlesssteel for strategic applications. MIDHANI manufacturesstainless steel of various types that broadly fall intoaustenitic, ferritic, duplex, martensitic and precipitation

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hardening stainless steel. Based on microstructure, stainlesssteels are categorized into five classes as shown in Table 1below.

Table 1 : Type of Stainless steels

Mechanical PropertiesType Stable Phase (typical)

0.2% Proof Tensile Elonga-Strength Strength tion(MPa) (MPa) (%)

Austenitic Austenite 200-380 500-800 45-60

Ferritic Ferrite 275-600 450-700 20-30

Duplex Austenite + ferrite 315-565 590-740 20-40

Martensitic Ferrite + Carbide 275-1900 520-2000 15-30

Precipitation Austenite orHardening Martensite +

precipitates 1200-1800 1380-1900 5-15

It is evident that there is significant overlap in mechanicalproperties of various types of stainless steels. The choice ofthe type of stainless steel for a particular application willobviously depend on strength and many other criteria suchas resistance to various types of corrosion, oxidationresistance, creep and fatigue strength and micro structuralstability at the operating temperature.

The metallurgical characteristics of these classes differsignificantly, but within a class the properties are similar. Thetypes of stainless steels in MIDHANI’S product mix andtheir salient qualities are discussed below.

3.1 Austenitic stainless steel

Austenitic stainless steel, being single-phase material,can be strengthened only by cold work apart from solidsolution strengthening. The presence of nickel improvesresistance to corrosion. Type 304 is the basic 18Cr8Niaustenitic stainless steel and is widely used. Resistance ofSS304 to pitting corrosion and stress corrosion cracking inchloride bearing environments is relatively poor. To improvethese characteristics, nickel and chromium levels areincreased and molybdenum is added up to 4%. Which resultsin another improved grade, SS316. Austenitic stainless steelswith higher Cr & Mo are designated as super austeniticstainless steels. Austenitic stainless steels are extensivelyused by almost all engineering industries. Because ofexcellent corrosion resistance, weldability and ease offabrication, austenitic stainless steels are used in chemical,pharmaceutical, marine, coastal environments, beverage andfood processing industries for piping, reactor vessels,equipment construction etc. Some grades of austeniticstainless steels are extensively used in gas turbine enginesand for biomedical implants.

For austenitic stainless steels such as MDN304, MDN310,MDN316, MDN321, MDN316Ti, MDN347 etc, with carbongreater than 0.03% melting is done in Electric Arc Furnace orInduction Furnace. Steels such as MDN304NAG, MDN304L,MDN304LN, MDN304L (N), MDN316L, MDN316L (N) withcarbon less than 0.03% or high nitrogen grades such asMDN202 (Equivalent 07X21G7AH5) are melted in AirInduction Melting or Vacuum Induction Refining furnacewith selective use of low carbon ferroalloys and carbon boil.The primary melted material is then secondary remelted inVacuum Arc Remelting or Electro Slag Remelting furnace.

Austenitic Stainless Steels such as MDN202 (Equivalent07X21G7AH5), MDN301 (Equivalent 07X16H6) andMDN321A (Equivalent 12X18H10T) find extensive use incryogenic applications. Nuclear sector uses steels such asMDN304NAG, MDN304L, MDN 304L(N), MDN316L(N),MDN08X18H10T, MDN12X18H10T and MDN347.

The melt route to be adopted will depend on the productspecification of the customer. Depending on the productspecification same grade may be processed by different meltroute. Some of the critical grades manufactured are discussed.

MDN316Ti modified (D9) finds extensive use in reactorcore application for clad and wrapper tubes. The specificationcalls for narrow range of chemical composition, low gaslevels, stringent ultrasonic requirement (1mm FBH) and finegrains. Ensuring high quality raw material during primarymelting, double vacuum melting (VIM + VAR), adequateforging reduction and homogenization of material fulfills therequirement.

MDN304 NAG (Nitric Acid Grade) specifies a corrosionrate of 5 mils per year (mpy) maximum. This steel is processedin vacuum induction refining furnace. Taking MIDHANI orcommercial pure iron ensures low sulphur and phosphorus.Carbon is lowered below 0.015% by carbon boil with additionof nickel oxide or iron oxide. In order to eliminate delta ferritethat increases corrosion, austenitic stabilizer such as nickel,manganese etc are kept on the higher end and ferritic stabilizersuch as chromium, silicon etc are kept on the lower end ofthe specified limits. The material is then remelted in ESR tofurther lower sulphur, silicon and oxygen. The material isthen forged and hot rolled to sheets and solution annealed.In this case the corrosion rate directly depends mainly on thecarbon content. Material so processed met the corrosion ratespecified.

MDN304L (N) and MDN316L (N) are low carbon grades,all the elements have been narrowed down and nitrogenlimits are specified, essentially to narrow the scatter, improvestrength and weldability. These steels are manufacturedthrough electric arc furnace to provide pure iron with lowsulphur and phosphorus. The hot liquid metal is transferredto Vacuum Induction Refining furnace where major alloyingelements are added and final aimed analysis is obtained. Thecast electrode is then remelted in ESR and subsequentlyprocessed by forging, hot rolling into bars, flats, plates orsheets.

High nitrogen grades such as MDN305N, MDN21-4Nwith nitrogen upto 0.4% have been manufactured. Nitrogenin these grades have introduced by high nitrogen ferroalloys.The solubility of nitrogen in austenitic is reduced by nickelbut increased by chromium and manganese. The primaryreason for adding nitrogen is that it is a very effective solidsolution strengthener. These properties of nitrogen makethem suitable for niche applications such as power generatorretaining rings, high strength bolts and super conductingmagnet housings.

3.2 Ferritic stainless steel

Ferritic stainless steels are constitutionally the simplestalloys containing iron and chromium in excess of 13% andare single-phase materials. Therefore unlike the martensiticgrades, ferritic grades cannot be hardened by heat treatment.Increasing the chromium content increases the corrosion andoxidation resistance, but tend to get embrittled due to theformation of embrittling phases such as sigma, chi and lavesphase. Some of the important ferrite stabilizing elements arechromium, silicon, molybdenum, tungsten and aluminum.

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They exhibit lower strength but have higher ductility /toughness. To increase the formability and corrosionresistance of ferritic stainless steels, chromium levels areincreased up to 30% with addition of molybdenum up to 4%and the concentration of interstitials (carbon and nitrogen)is reduced. Such steels with increased strength andtoughness and enhanced corrosion resistance and weldabilityare known as super ferritic stainless steels. Typicalapplications are food-handling equipment, heat exchangers,piping systems, furnace parts, automobile exhausts andarchitectural trims.

MIDHANI manufactures only limited grades of ferriticstainless steel such as MDN430, MDN430 modified andMDN446. These steels are primary melted in InductionMelting furnace and remelted through Electro Slag Remeltingfurnace. MDN430 and MDN446 find application in aerospace.MDN430 modified is specially manufactured for BARC. Thisparticular grade requires high strength and toughness to bemet. Composition was suitably modified with the addition ofaustenitic stabilizing elements such as nickel and nitrogen tomake the steel heat treatable. The new steel has dual phase(austenite and martensite) structure in hardened condition.Tempering is carried out close to AC1 to get primary ferriteand transformed ferrite and carbide matrix. This results in anearly 25 % higher strength coupled with 60% increase intoughness compared to unmodified steel. It finds use instepper motor as rotor because of its magnetic properties.

3.3 Duplex stainless steel

Duplex stainless steels contain a mixture of ferrite andaustenite. The two-phase mixture also leads to a markedrefinement in the grain size of both austenite and ferrite.Duplex stainless steel is twice as strong as common austeniticsteels. They have significantly higher corrosion resistancethan all other grades of stainless steels.

The ferrite and austenite stabilizing elements are balancedsuch that the composition falls in the α + � phase field of thephase diagram. Steels with higher chromium and molybdenumconcentrations have been developed to enhance pittingcorrosion resistance. In these grades ferrite-stabilizingelements are balanced using a higher nickel and nitrogenconcentration (austenitic stabilizing) in order to maintainabout equal amounts of ferrite and austenite. Steels thathave pitting index greater than 40 are called super duplexstainless steels. These steels have better formability andweldability than ferritic family. High strength levels coupledwith excellent resistance to stress corrosion cracking andlocalized corrosion make duplex stainless steels favoritematerials for marine, chemical and food processingequipments.

MDN329 and MDN312, manufactured in MIDHANI fall induplex stainless steel category. Higher nitrogen gradeMDN32950 have been manufactured and supplied for HeavyWater Project – Manuguru. This steel has high strength andhigh corrosion resistance compared to MDN329. The ferriteto austenite ratio is approximately 60:40 and belongs to superferritic stainless steel category.

3.4 Martensitic stainless steel

The constant search for high strength stainless steelshas resulted in a composition that is austenite at hightemperature and transforms into martensite on cooling. Thedefinition of transformable stainless steel is that it is steelthat has an Ms – Mf range above room temperature. In

stainless steels, the ferrite and bainite transformations are soretarded that these structure are not formed even at veryslow cooling rates, which means that a uniform martensitestructure is produced. The transformable category isattractive because the transformation to martensite producesa high initial strength and it is therefore, only necessary tomaintain this strength at a reasonable tempering temperatureto obtain a satisfactory combination of mechanical properties.

Martensitic stainless steels derive strength from thetransformation of face centered cubic austenite to bodycentered tetragonal martensite. Typical heat treatmentconsists of austenisation at a temperature high enough todissolve carbides followed by quenching (oil or water whendealing with thick sections). The untempered martensiticstructure is strong but lacks in toughness and ductility. Themartensite is tempered between 600–750oC to optimize themechanical properties. The strength of martensite almostexclusively depends on the carbon content. Depending onthe properties required, the carbon and chromium levels areselected. Elements such as molybdenum, tungsten andniobium are added to modify the tempering characteristics.Since chromium content is partly utilized in the formation ofchromium rich carbides, the corrosion resistance is slightlyreduced in higher carbon grades.

Too much chromium addition will lead to the formation ofδ-ferrite, an undesirable phase. Nickel up to about 2% isadded in high chromium martensitic grades to suppress theformation of δ-ferrite. Most low carbon martensitic gradespossess high strength, good toughness, moderate corrosionresistance and good oxidation resistance. High carbonmartensitic stainless steels can be hardened and tempered tohardness levels of 60 HRC.

Low carbon varieties of martensitic stainless steels areused for various parts in steam turbines, gas turbines andpetrochemical equipment. The high carbon grades are usedfor knives, surgical instruments, valves, ball bearings andgears.

A number of martensitic stainless steels are manufacturedat MIDHANI. The most important among them are MDN403,MDN410, MDN420, 12Cr13, 20Cr13, 30Cr13, 35Cr13, MDN121,MDN122, MDN123 and MDN440C.

MDN403 is a low carbon martensitic stainless steel thatfinds applications as end fitting in Pressurized Heavy WaterReactor due to its high strength and impact toughness,adequate resistance to corrosion, wear, erosion and oxidationresistance; and thermal coefficient of expansion close tozirconium alloy pressure tube. The steel for end fittingspecifies a limit of cobalt of 0.025% and copper 0.06%. Theproduct after heat treatment is expected to have high strengthand toughness. This steel is produced in MIDHANI throughElectric Arc Furnace, in which pure iron of low sulphur andphosphorus and also Co & Cu requirement is achieved. Thehot liquid metal is then transferred to Vacuum InductionRefining furnace for obtaining low gas levels. The secondarymelting is carried out in ESR. The outcome is a material thathas low trace element such as sulphur, phosphorus, oxygen,hydrogen, cobalt and copper and a sound and dense material.Thermo-mechanical working and adequate forging reductionensures a fine-grained, homogenous material. These forgingsare then heat treated in calibrated furnace to obtain optimumstrength and toughness.

Another important steel is MDN440C, which is a veryhigh carbon martensitic stainless steel that is capable ofdeveloping extremely high level of hardness (over Rc 60) andwear resistance. This steel maintains excellent dimensionalstability after heat treatment and finds application as critical

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valve parts and control systems in aerospace and also asbearing component in nuclear sector. MIDHANI manufacturedthis grade for bearing application. One of the mainrequirements is that the microstructure should not have anycoarse primary carbide in the form-banded structure. Theprimary melting is carried out in air induction furnace andsubsequently remelted in ESR. The ingot size is maintainedsmall to minimize segregation because this alloy is highlysegregation prone as the carbon content is in excess of 1%.The ingot is forged with adequate reduction so that thecarbide network, if any, will be broken. The result is a finedistribution of carbides in the matrix of ferrite.

One of the grades made in MIDHANI is MDN 59, whichfinds extensive use in gun carriages. Over the years it wasobserved that apart from controlling chemical compositionthe ratio of Niobium and Carbon played an important role inaffecting the impact strength. The Fig. 1 shows theseobservations schematically.

3.5 Precipitation hardening stainless steel

The need for the development of Precipitation Hardening(PH) Stainless Steels is the requirement of corrosion resistantsteels with strength and toughness levels superior tomartensitic stainless steels. This is achieved by suitablealloying additions and appropriate heat treatment to induceprecipitation hardening in austenitic and martensitic stainlesssteels. The primary differences between martensitic stainlesssteels and the precipitation hardening steels are: (a) in as-quenched condition, the martensitic stainless steels are hardand brittle while PH stainless steels are soft and ductile insolution treated condition (b) on tempering, the strengthdecreases and toughness increases in martensitic stainlesssteels whereas on ageing, the strength increases withoutsignificant decrease in toughness in PH stainless steels.

Precipitation hardening or age hardening provides one ofthe most widely used mechanisms for strengthening ofstainless steel. This category of stainless steels derivesstrength due to the precipitation of fine coherent precipitates.The major advantage when compared to martensitic stainlesssteels is that the PH steels can be fabricated in annealedcondition and subsequently precipitation hardened at lowertemperatures. Such treatment eliminates the need for fastcooling and attendant problems like warpage and crackingdue to martensitic transformation. Both austenitic andmartensitic stainless steels can be altered to precipitation

hardening grades by the addition of precipitate formingelements such as aluminium, titanium, molybdenum andcopper. By suitable compositional balance, PH steels can bemade to austenitic, semi austenitic and martensitic types. PHstainless steels in particular have high strength, goodtoughness and corrosion resistance both at ambient andelevated temperatures (temperatures upto 350°C), fabricabilityand weldability. PH stainless steels find extensive use inaircraft structures, landing gears, armament, aerospace andnuclear and other engineering applications.

MIDHANI has developed several grades of precipitationhardening stainless steels for aerospace, defence and nuclearapplications. Notable among them are MDN13-8, MDN15-5,MDN17-4, MDN17-7, MDN455, MDN59, MDN60 andSuperfer A286 Precipitation Hardening Stainless Steels. ThePH steels finally derive their strength through an agingtreatment, which results in precipitation of intermetallicphases in a low carbon martensite matrix. Some of the alloys,type and their strengthening precipitation are shown belowin Table 2.

Table 2 : Strengthening phases in different PH steels

Grade Type Strengthening Precipitation

17-7 Semi austenitic BCC NiAl

17-4 Martensitic FCC Epsilon copper

13-8 Martensitic BCC NiAl

15-5 Martensitic FCC Epsilon copper

Based on the matrix structure after solution treatment, PHstainless steels are divided into three categories viz.,martensitic, semi-austenitic and austenitic.

In austenitic class, the matrix is a solid solution of ironcontaining transition elements like manganese, cobalt andnickel. MIDHANI manufactures Superfer A286 steel in thisclass. Precipitation reactions are induced by the presence Alwith Ti that forms effective precipitates in the iron basedsolid solution. Superfer A286 is a high strength, heat andcorrosion resistant (upto 700oC) alloy. The alloy is also usedfor low temperature applications requiring a ductile, non-magnetic high strength material at temperature ranging fromabove room temperature down to –196oC. This alloy isproduced by Vacuum Induction Melting and subsequentlyremelting either through Electro Slag Remelting or VacuumArc Refining furnace. This alloy is easily formed in solutiontreated condition like any other high strength austeniticstainless steel. It finds application in high temperaturefasteners, jet engine and gas turbine components, cryogenicdevices, nonmagnetic oil well equipments, springs etc.

In Martensitic and Semi Austenitic, substitutionalelements alloyed produces precipitation hardening. The effectof these elements on precipitation hardening variessignificantly. Many high strength precipitation hardeningmartensitic steels are so alloyed that the strengthening isderived primarily from the precipitation of coherent orderedNi-Al as in MDN13-8 grade or copper rich precipitates as inMDN15-5 and MDN17-4 grades. Depending on the ageingtemperature and time, the size and sometimes the shape ofthe precipitates vary. A variety of age hardening treatmentsis employed for martensitic PH steels and consequently awide range of strength-toughness combinations areattainable in the same grade. In most of the PH martensiticstainless steels, precipitation of reverted austenite occurswhen aged at higher temperatures or for longer times at lowertemperatures. Various types of carbides like MC, M7C3 andFig.1 : Higher Nb/C ratio improves the impact toughness

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M23C6 also precipitate, though their contribution to overallstrength is not very significant.

MDN 174 is widely used for components in nuclear powergeneration. A limit of 0.04% cobalt is specified. This grade isdifficult to process and is crack prone. Primary melting iscarried out in electric arc furnace. Qualified raw materials aretaken to ensure low cobalt and phosphorus. The electrodeis then remelted in ESR to improve the cleanliness byreduction of oxygen, sulphur and aluminium. An optimumthermo-mechanical treatment and forging reduction ensuresa uniform and fine-grained structure. This chemicalcomposition of this grade is such that some amount of deltaferrite forms. This affects the transverse impact toughness.

MDN138 and MDN155 are the improvement over Type174. Higher nickel and lower chromium content reduces oreliminates the formation of delta ferrite. MDN138 is widelyused for ball screw manufacturing. Lower delta ferrite ensuresa homogenous material that in turn ensures isotropicproperties. The specification for ball screw calls for minimumimpact toughness of 2.8 Kg.m in H1000 condition with ahardness of 40HRc. None of the international specificationspecifies a impact toughness requirement. MIDHANI tookup the challenge. The material is double vacuum melted(VIM+VAR) to obtain a material that is clean, low in gascontent, sound and homogenous. This material is forged andhot rolled in a controlled temperature range to result in afine-grained structure. Solution annealing and ageing cycleare carried out in well-calibrated furnace. After initialchallenges faced MIDHANI’S MDN138 is consistentlymeeting the required specificational requirement. A optimumNi/Al ratio ensured both strength and toughnesscombination. MIDHANI has made interesting observationswith respect to variations of mechanical properties in case ofMDN 13-8 PH steels. The observations in the graphical formsare shown in Figs. 2 to 6.

Fig. 2 : Impact toughness consistently higher than specified

Fig. 3 : Proof and Ultimate Tensile Strength well above thespecified minimum

Fig. 4 : Effect of Ni/Al ratio on ultimate tensile strength

Fig. 5 : Effect of Ni/Al ratio on 0.2% Proof strength

Fig. 6 : Effect of Ni/Al ratio on impact toughness

As can be observed the Nickel Aluminum ratio plays animportant role in determining strength and toughness. Alsoconsistent results were observed in most of the heats.

The semi austenitic types, like MDN17-7, grade, arestrengthened by precipitation of ordered coherent Ni-Alprecipitate and incase of MDN60 due to copper richprecipitates. The achievable strength levels in semi austeniticgrades critically depend on heat treatment conditions. Thebest strengthening effect is achievable by minimizing theretained austenite content.

All the grades of PH stainless steels have good corrosionresistance in most of the media. The martensitic gradesdevelop mild rust and pits when exposed to marineenvironment. Semi-austenitic grades possess corrosionresistance similar to the popular austenitic stainless steelgrade AISI 304. Resistance to stress corrosion cracking canbe improved significantly if all PH stainless steels are usedin slightly over aged condition. The austenitic PH stainlesssteels have excellent resistance to sulphuric and phosphoricacids. They perform satisfactorily in seawater and alkaline.

The martensitic and semi austenitic grades are used inapplications demanding high specific strength and corrosionresistance. Airframe skin and structural parts, undercarriageparts, honeycomb panels, chemical equipment, fluid controlsystems etc. are some of the examples.

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Another important precipitating hardening steel thatneeds mention is Maraging steel. This steel does not fallunder the stainless category, as chromium that gives thecorrosion resistant property to steel is not present. Maragingsteel is manufactured at MIDHANI. Maraging steels are afamily of ultra high strength steel coupled with excellentfracture toughness. These steels have good hot and coldworkability, fabricability, and machinability and are easilyheat treated without problems of distortion and cracking. Inaddition, these steels possess very high specific strengthcomparable to titanium alloys. These characteristics ofMaraging steels make them ideally suited for aerospaceapplication. MIDHANI is presently manufacturing the 250,300 and 350 grades of 18% Ni Maraging steels. The MDN250 grade has been widely used as motor casing hardwarefor Polar Satellite Launch Vehicle and Geo Stationary LaunchVehicle rockets as well as in the missiles developmentprogramme. The steels have been developed by the doublevacuum-melting route (VIM + VAR) in order to achieve avery high level of cleanliness and low levels of dissolvedgases. Close controls are exercised during hot working andheat treatment in order to achieve the desired microstructureand properties.

4. Quality assurance system

In the context of production of materials for strategicsector, the concept of Quality Assurance assumes greatestrelevance. Since there is no scope or mechanism forreplacement of defective parts during usage, it becomesimperative that components perform reliably as per the design.

In this context, one can appreciate the need for assuringquality from design through production to testing. InMIDHANI a sound quality assurance system has beenevolved over the years. ISO 9001 - 2000 certification for thecompany has been significant milestone in MIDHANI’Sintended voyage to achieve the goal of TQM and stand uptoand face challenges of new material requirements of thecountry for the 21st century. Today MIDHANI’S QA systemhas the approval of Directorate of Technical Developmentand Production (Air), Director General of Civil Aviation(DGCA), Director General of Quality Assurance andDepartment of Atomic Energy.

5. Conclusion

The fast pace of developments taking place in the countryin the area of nuclear power, thermal power, marine,aeronautical and aerospace has given MIDHANI enoughopportunity to showcase its technical competencies. Thishas resulted in widening the product base of MIDHANI.Midhani is committed for development and productionisationof advanced stainless steels for critical and strategic sectors.

References

1. Davies R G and Magee C L, Proc Symp TMS – AIME HeatTreatment Committee, (eds) Kot R A and Morris J W, Structureand Properties of Dual Phase Steels, (1979)

2. Marshall P, Austenitic Stainless Steels, in Elsevier Applied SciencePublisher, New York, (1984)

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Appendix : Midhani’s Stainless Steels Grades and their Equivalents

Content, wt. % - maximum unless range is mentionedMidhani grade Russian French American British German

C Si Mn P S Cr Ni Mo V Co W Others

AUSTENITIC STAINLESS STEELS

MDN 168 316 0.065- 0.40- 1.3- 0.02 0.015 16.6- 8.1- 1.85- 0.2 N: 0.09-0.11, B: 0.002,0.075 0.50 1.5 16.9 8.3 1.95 Ti+Nb+Ta: 0.1,Cu: 0.5

MDN 202 07X21G7 202 0.07 0.5 6-7 0.03 0.01 19.5- 5.5- N: 0.18-0.25AMS 21.5 6.5

MDN 301 07X16H6 Z12CN17.08 0.09 0.8 0.09 0.02 0.02 16-18 6-7 N:0.01

MDN 304 Z6CN18.09 304 304S15 X5CrNi18.10 0.08 1.00 2.00 0.045 0.03 18-20 8-10.5 N: 0.10

MDN 304 L Z2CN18.10 304L 304S12 X2CrNi19.11 0.30 1.00 2.00 0.045 0.03 18.0- 8-12.0 N: 0.1020.0

MDN 304LN 304LN 0.03 0.75 2.0 0.040 0.030 18-20 8-12 - - - - N 0.10-0.16

MDN 304L (N) 304L 0.30 1.00 2.00 0.045 0.03 18.0- 8-12.0 N: 0. 6-0.820.0

MDN 304NAG 304L 0.025 0.35 2.0 0.030 0.01 18-20 10-12 0.10 Al: 0.05,B: 0.0015,Ti :0.05, Cu: 0.10,

N: 0.05

MDN 305LN 0.015- 0.65 1.6 0.020 0.020 17-19 12-14 - - - - N: 0.10-0.200.025

MDN 308L 0.02 0.30 1.0- 0.025 0.01 19.5- 9-11 0.52.0 22.0

MDN 310 Z12CNS 310 310S24 X15 0.20 2-3 1.5 0.040 0.03 24-27 18-2125.20 CrNiSi25.20

MDN 310S 310S 0.08 1.50 2.00 0.045 0.030 24-26 19-22

MDN 316 Z8CND17.13 316 0.080 1.00 2.00 0.045 0.03 16-18 10-14 2.00-3.00 N: 0.10

MDN 316L Z2CND17.13 316L 0.03 1.0 2.0 0.04 0.02 16-18 10-14 2-3 N: 0.10

MDN 316L (N) 316L 0.02- 0.5 1.6- 0.03 0.015 17-18 12- 2.3-2.7 0.05 N: 0.055 - 0.085,0.03 2.0 12.5 Cu: 1.0Nb: 0.05,B:

0.020, Ti:0.05

MDN 316 L-VM 316L 0.025 0.75 2.0 0.025 0.010 17-19 13-15 2.25-3 N: 0.10

MDN 316Ti Z8CNDT 316Ti 0.08 1.0 2.0 0.040 0.03 16-18 12-14 2-3 Ti: 5. C/0.70, N: 0.1017.12

MDN 316Ti S38660 0.035- 0.50- 1.65- 0.02 0.01 13.5- 14.5- 2.0-2.5 Ti: 5.C/7.5C, Nb: 0.05,MOD. 0.05 0.75 2.35 14.5 15.5 N: 0.005B: 0.01-0.020

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Appendix : Midhani’s Stainless Steels Grades and their Equivalents—contd.

Content, wt. % - maximum unless range is mentionedMidhani grade Russian French American British German

C Si Mn P S Cr Ni Mo V Co W Others

MDN 317L Z2CND 317L 317S12 X2CrNiMo 0.080 1.00 2.00 0.045 0.03 18-20 11-15 3.00- N: 0.10 max19.15 18.16.4 4.00

MDN 321 08X18 Z6CNT18.10 321 321S12 X6CrNiTi18.10 0.08 0.80 2.00 0.035 0.02 17-19 9-11 Ti: 5C-0.80, N: 0.11,H10T

MDN 321A 12X18 Z10CNT 321 321S12 X10CrNiTi 0.070.12 0.80 2.00 0.035 0.02 17-19 9-11 Ti: 5C-0.80, N:0.11,H10T 18.10 18.10

MDN 330W 0.07- 0.40 1.0- 0.012 0.015 15-17 24-27 5.5- 0.70- N: 0.10-0.200.11 2.0 7.0 1.0

MDN 331W 0.08- 0.60 1.0- 0.018 0.018 15-17 24-27 5.5-7.0 N: 0.10-0.200.12 2.0

MDN 347 Z6CNNb 347 347S17 X6CrNiNb 0.08 1.0 2.0 0.040 0.030 17-19 9-13 N: 10C – 1.018.10 18.10

MDN 21-4N 0.48- 0.25 7-10 0.045 0.030 20-22 3.25- - - - - N: 0.38-0.500.58 4.50

MDN 904L 904L 0.02 0.5 2.0 0.02 0.015 19-23 23-28 4-5.0 Cu: 1.0-2.0

SUPERFER 0.03 0.3- 0.40- 0.015 0.015 20-23 32-35 - - 0.015 Al: 0.15-0.45, Ti: 0.6,800 L max 0.7 1.0 N: 0.03 Cu: 0.075

SUPERFER 0.06- 1.0 1.5 19-23 30-35 Al:1, Ti:0.85-1.20800H 0.1

SUPERFER 286 Z6NCTD 660 0.08 1.00 1-2 0.030 0.015 13.5- 24-27 1-1.5 0.10- Ti: 1.90-2.30, Al: 0.35,V25.15 16 0.50 B: 0.001-0.010

MARTENSITIC STAINLESS STEELS

MDN 121 X22CrMoV121 0.18- 0.10- 0.30- 0.03 0.02 11- 0.30- 0.8- 0.25-0.24 0.50 0.80 12.50 0.80 1.20 0.35

MDN 122 XM22 X10CrNiMoV 0.08- 0.30 0.60- 0.045 0.03 11.5- 2.2- 1.6- 0.3- N: 0.02-0.041222 0.12 0.80 12.5 2.5 1.9 0.4

MDN 123 616 0.20- 0.50 0.50- 0.025 0.025 11.0- 0.50- 0.90- 0.20- 0.90-0.25 1.00 12.50 1.00 1/25 0.30 1.25

MDN 124 0.11- 0.12 0.40- 0.015 0.005 10.20- 0.70- 1.00- 0.15- 0.95-1.05 Al: 0.010,0.13 0.50 10.80 0.80 1.10 0.25 Nb: 0.04-0.06

N: 0.045-0.06

MDN 134 0.05 0.60 1.00 0.015 0.015 12.5- 3.5- 0.4- Al: 0.0514.0 4.5 0.7

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Appendix : Midhani’s Stainless Steels Grades and their Equivalents—contd.

Content, wt. % - maximum unless range is mentionedMidhani grade Russian French American British German

C Si Mn P S Cr Ni Mo V Co W Others

MDN 403 Z12Cr13 403 0.15 0.50 1.00 0.04 0.03 11.5-13.0

MDN 410 Z20Cr13 410 410S21 0.15 0.50 1.00 0.04 0.03 11.5- 0.7513.5

MDN 420 420 420S37 0.15 1.00 1.00 0.04 0.03 12-14

MDN 12Cr13 420 0.09- 1.00 1.00 0.02 0.02 11.50-0.15 14.00

MDN 20Cr13 420 0.17- 0.10- 0.30- 0.03 0.02 12.5- 0.300.22 0.50 0.80 14.00

MDN30Cr13 Z30Cr13 420 0.25- 1.00 1.00 0.04 0.03 12.00-0.34 14.00

MDN 35Cr13 420 0.3-0.4 1.00 1.00 0.03 0.025 12-14 0.50

MDN 40Cr13 420 0.35-0.45 1.00 1.00 0.03 0.025 12-14 0.50

MDN 70Cr13 420 0.72 0.2-0.5 0.5-0.8 0.025 0.020 12.5 13.7 0.5 Cu: 0.3 max

MDN 431 Z15CN17.03 431 431S29 X20CrNi17.2 0.20 1.00 1.00 0.04 0.03 15-17 1.25-2.50

MDN 431A Z15CN17.03 431 S80 0.12- 1.0 1.0 0.030 0.025 15-18 2.0- 0.30 0.20 0.05 0.05 Ti: 0.05Sn: 0.02,0.20 3.0 Nb: 0.05, Cu: 0.30

MDN 14X17H2 431 0.11- 0.80 0.80 0.035 0.020 16-18 1.5-2.50.17

MDN 25X17H2BW 431 0.22- 0.30- 0.30- 0.020 0.015 16.3- 2.30- Nb: 0.05-0.1, Cu:0.250.28 0.70 0.70 17.7 2.80

MDN 440A 440A 0.60- 1.00 1.00 0.04 0.03 16.0- 0.750.75 18.0

MDN 440B 440B X90CRMoV18 0.75- 1.00 1.00 0.04 0.03 16-18 0.750.95

MDN 440C Z100CD17 440C 0.95- 1.00 1.00 0.04 0.03 16-18 0.751.20

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Appendix : Midhani’s Stainless Steels Grades and their Equivalents—contd.

Content, wt. % - maximum unless range is mentionedMidhani grade Russian French American British German

C Si Mn P S Cr Ni Mo V Co W Others

DUPLEX STAINLESS STEELS

MDN 312 312 0.015 0.9 0.5-2.5 0.04 0.03 28-32 8-10.5 0.5 Cu: 0.5, N: 0.08

MDN 329 329 0.03 0.60 2.00 0.035 0.010 26.0- 3.5-5.2 1.00- N: 0.15-0.3829.0 2.50

FERRITIC STAINLESS STEELS

MDN 430 430 0.12 1.00 1.00 0.04 0.03 16-18 0.75

MDN 430 M 09X17HW 0.09 0.4- 0.5 0.03 0.025 15.6- 0.8-0.8 17.6 1.2

MDN 446 446 0.20 1.00 1.50 0.04 0.03 23-27 0.75

PRECIPITATION HARDENING STAINLESS STEELS

MDN 59 STA59 0.07 0.70 1.00 0.04 0.02 13.2- 5-6 1.2-2 1.20- Nb: 0.20-0.5014.7 2.00

MDN 60 STA60 0.04- 0.70 0.80- 0.04 0.02 15.3- 5.20- 1.20- 1.40- Nb: 0.20-0.500.07 1.80 16.3 6.00 2.00 2.10 Ti: 0.15-0.30

MDN 138 PH XM13 0.05 0.10 0.20 0.10 0.008 12.25- 7.50- 2.00- Al: 0.9-0.15,13.25 8.50 2.50 N: 0.01 max

MDN 155 PH XM12 0.07 1.00 1.00 0.03 0.015 14.0- 3.50- 0.50 2.5- Al: 0.15 max15.5 5.50 4.5 Nb: 5C-0.45

N: 0.035 max

MDN 174 PH 630 0.07 1.00 1.00 0.04 0.03 15.50- 3.0- 3-5 Nb: 0.15-0.4517.50 5.0

MDN 177 PH 631 0.09 1.00 1.00 0.04 0.03 16-18 6.5- Al: 0.75-1.507.75

MDN 455 XM16 0.03 0.50 0.50 0.015 0.015 11- 7.50- 0.50 1.50- Nb: 0.10-0.5012.50 9.50 2.50 Ti: 0.90-1.40

MDN 11 – 10 PH 03X11H10 - 0.03 0.15 0.10 0.01 0.01 10- 9- 1.8- Al: 0.20, Nb: 0.15,M2T 11.3 10.3 2.3 Ti:0 .6-1.0, Cu: 0.3,

B: 0.002, Zr: 0.06,N: 0.1