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Alloy Steels

Introduction

Carbon steels usually contain less than 1,2% carbon and small quantities of manganese, copper, silicon, sulfur, and phosphorus    Alloy steels are carbon steel with other metals added specifically to improve the properties of the steel significantly.  Stainless steel are considered a separate group.
Plain carbon steel is produced with a wide range of mechanical properties with comparatively low cost.  To extend the range of properties of steel, alloys have been developed.   The benefits resulting include

  • The maximum UTS is increased
  • Thick sections steels are available with high hardness throughout the section
  • More controllable quenching with minimum risk of shape distortion or cracking
  • Improved impact resistance at high temperature range
  • Improved corrosion resistance
  • Improved high temperature performance

The principle elements that are used in producing alloy steel include nickel, chromium, molydenenum, manganese, silicon and vanadium.   Cobalt, copper and lead are also used as alloying elements.

Effect of alloying elements

Elements may encourage formation of graphite from the carbide.  Only a small proportion of these elements can be added to the steel before graphite forms destroying the properties of the steel, unless elements are added to counteract the effect.   Elements which encourage the formation of graphite include silicon, cobalt, aluminium and nickel

Alloying elements may go into solid solution in the iron, enhancing the strength.    Elements which go into solid solution include silicon, molybdenum, chromium, nickel and magnesium.

Hard carbides (cementite) associated with iron and iarbon may be formed with alloying elements.    Elements which tend to form carbides include chromium, tungsten, titanium, columbium, vanadium, molybdenum and manganese.

Elements which stabilise austenite include manganese, nickel, cobalt and copper.    These increase the range over which austenite is stable e.g. by lowering the eutectoid temperature, and this retards the separation or carbides.   If these alloys are present is certain high levels the austenite phase is dramatically reduced and the ferrite ( α) phase exists down to ambient temperatures e.g.18% chromium .

Elements which tend to stabilise ferrite include chromium, tungsten, molybdenum, vanadium and silicon.  They reduce the amount of carbon soluble in the austenite and thus increase the volume of free carbide in the steel at a given carbon content.  The effectively reduce the austenite ( γ ) phase by raising the eutectoid temperature and lowering the peritectic temperature

Intermediate compounds with iron may be formed e.g. FeCr

Alloying elements may adjust the characteristics such as eutectoid content, quenching rate which produces bainite or martensite.






Relative effect alloying elements

The combined effect of alloying elements results from many complex interactions resulting from the processing history, the number and quantities of constituents, the heat treament, the section shape etc etc..  Some basic rules can be identified.

Nickel has reduced carbide forming tendency than iron and dissolves in α ferrite.
Silicon combines with oxygen to form nonmettalic inclusions or dissolves in the ferrite.
Most of the manganese in alloy steels dissolves in the α ferrite .   Any manganese that form carbides result in (Fe,Mn)3C.
Chromium spreads between the ferrite and carbide phases the spread depending on the amount of carbon and other carbide generating elements present.
Tungsten and molybdenum form carbides if sufficient carbon is present which has not already formed carbides with other stronger carbide forming elements.
Vanadium , titanium, and colombian are strong carbide forming elements and are present in steel as carbides.
Aluminium combines with oxygen and nitrogen to form Al2O and AlN




Notes on alloying elements

Aluminium

Range 0-2%..This increases resistance to oxidation and scaling, aids nitriding and restricts grain growth.

Chromium
Range 0,3% to 4% ..Improves wear, oxidation and scaling resistance and hardenability but increases grain growth and reduces ductility
Range 12% to 30 % ...For production of stainless steels with nickel .

Cobalt

Enhances air hardenability and reduces scaling.  In tool steels allows use at high temperatures without softening.
Range (8% to 10%) to produce hard tough cutting steels (HSS).


Copper

Range 0,2% to 0,5%... Improves corrosion resistance and yield strength of low alloy steels.

Lead

Range 0 to 0,25% improves machinability in non-alloy low carbon steels.   Reduces strength and ductility.

Manganese

Range 0,3% to 1,5% alway present in steels to reduces the negative effects of impurities carried out forward from the production process e.g sulphur embrittlement.
Promotes the formation of stable carbides in quenched-hardened steels.   Alloys containing manganese are pearlitic.
Up to 1% acts as hardening agent and from 1% to 2% improves strength and toughness.
Alloys containing more than 5% are non-magnetic.
Alloys containing large proportions of up to 12,5% manganese have the property that they spontaneously form hard skins when subject to abrasion. (self-hardening)

Molybdenum

Range 0,3% to 5%. Stabilises carbides and promotes grain refinement and increases high temperature strength, creep resistance and hardenability.  Useful in cutting tool materials.   In nickel-chromium steels reduces temper embrittlement.

Nickel

Range 0,2% to 5% Improves strength, toughness, and hardenability without seriously affecting the ductility.   Encourages grain refinement    Can graphitise carbide resulting in softening.
Nickel and chromium together have opposing properties and are used together to advantage in nickel-chrome steels.   The resulting steels have their advantages combined and their undesirable features cancel each other
At 5% nickel provides high fatigue resistance.    When alloyed at higher proportions significant corrosion resistance results and at 27% a non magnetic stainless steel results.

Phosphorus

Range 0-0,05% Residual element from production process (Casting). Results in weakness in the steel.   Kept below 0,05%.   Can improve machinability and in larger quantities improves fluidity in cast steels.

Silicon

Range 0,2% to 3%.   Used mainly in production of cast iron causing graphitisation and is not used in large proportions in high carbon steels.   Up to 0,3% improves fluidity of casting steels without the weakening effect of phosphorus   Up to 1% improves the heat resistance of steels.   At 3% improves strength and hardenability.   Acts as a de-oxidiser.
Also used to improve magnetic properties of soft magnetic materials used in laminations for transformers and motor stators and rotors.

Sulphur

Range up to 0,5% Residual impurity from production process.  Weakens steel and additional process are used to remove sulphur.  Neutralised by the presence of manganese.   Sometimes added to low carbon steels to improve machinability with the accepted penalty of reduced strength.   Reduces ductility and weldability.

Titanium

Strong carbide forming element.  In range 0,2% to 0,75% it is used in maraging steels to make them age hardening with resulting high strength.  Stabilises austenic stainless steel.

Tungsten

Forms hard stable carbides and promotes grain refining with great hardness at high temperatures.  The main alloying element in high speed tool steels.    Constituent in permanent magnet steels.

Vanadium

Carbide forming element and deoxidiser used together with nickel and or chromium to increase strength   Improves hardenability and grain refinement and combines with carbon forming wear resistant structure.  Is used as a deoxidiser in casting steels to reducing blowholes and increasing hardness and strength.  Vanadium is used with in high speed steel based on pearlitic chromium. "Improves fatigue properties of hardened steels



Stainless Steels

Stainless steels are steels with a high degree of corrosion resistance and chemical resistance to most a wide range of aggressive chemicals.  The corrosion resistance is mainly due to their high chromium content.    Stainless steels normally have more than 12% chromium.  Chromium makes the surface passive by forming a surface oxide film which protects the underlying metal from corrosion.   In order to produce this film the stainless steel surface must be in contact with oxidising agents.   Stainless steels are classified as Austenitic, Martensitic or Ferritic.

Nickel added to stainless steel improves corrosion resistance in neutral or weak oxidising environments and also improves ductility and formability by enabling the (FCC) crystal structure to be retained at normal temperatures.

Molybdenum added to stainless steel improves corrosion resistance in the presence of chloride ions.

Aluminium improves high temperature scaling resistance.

Austenitic Stainless Steels

These are usually alloy containing three main elements Iron Chromium and Nickel (6% to 22%). These steels cannot be hardened by heat treatment. They retain an austenitic structure at room temperature and are ductile and have good corrosion resistance compared to ferritic stainless steel. They are at risk of intergranular corrosion unless heat treated to modify their chemical composition.

Ferritic Stainless Steel

This steel normally contains 11% to 30% chromium with a carbon content below 0,12%.   Other alloying elements are added to improve its corrosion resistance or other characteristics such as machinability.   Because of the low carbon content ferritic stainless steels are not normally considered heat treatable. However there is some hardness improvement resulting from quenching from high temperatures.  The carbon and nitrogen content of these steels must be maintained at low levels for weldability , ductility and corrosion resistance.

Martensitic Stainless Steels

These steels contain 12% to 17% chromium with 0,1 to 1% Carbon. They can be hardened by heat treatment in the same way as plain carbon steels. Very high hardness values can be obtained for carbon levels approximately 1% using correct heat treatment. Small amounts of other alloying elements may be included to improve corrosion, resistance, strength and toughness.



Maraging Steels

Maraging steels are a class of high-strength steel with a low carbon content and the use of substitutional (as opposed to interstitial) elements to produce hardening from formation of nickel martensites. The name maraging has resulted from the combination of mar(tensite) + age (hardening)

Maraging steels contain 18% nickel, along with a amounts of molybdenum, cobalt, and titanium and aluminium, and almost no carbon.   These alloys can be strengthened significantly by a precipitation reaction at a relatively low temperature.   They can be formed and machined in the solution-annealed condition but not without difficulty.   Weldability is excellent.   Fracture toughness of the maraging steels is considerably higher than that of the conventional high-strength steels.

Maraging steels are hardened by a metallurgical reaction that does not involve carbon.   Maraging steels are strengthened by intermetallic compounds such as Ni 3Ti and Ni 3Mo which precipitate at about 500C.   The carbon content provides no real benefit and is kept low as possible in order to minimise the formation of titanium carbide which can adversely affect mechanical properties.    Toughness is superior to all low alloy carbon steels of similar strength, particularly the low temperature toughness.   These steels are easy to machine and heat treat, so some cost savings result in component production to compensate for the high cost of the steel.

A high strength maraging steel (extrusion section MIL-S-46850 grade 300) can have a 0,2% proof stress of 1930MPa and Ultimate Tensile strength of 2068MPa with an elongation of 4%



High Strength Low Alloy Steels (HSLA)

High strength low alloy (HSLA) steels are a group of low carbon steels that utilise small amounts of alloying elements to attain yield strengths in excess of 550 MPa in the as-rolled or normalised conditions.   These steels have better mechanical properties than as rolled carbon steels, largely by virtue of grain refining and precipitation hardening.   Because the higher strength of HSLA steels can be obtained at lower carbon levels, the weldability of many HSLA steels is at least comparable to that of mild steel.   Due to their superior mechanical properties, they allow more efficient designs with improved performance, reductions in manufacturing costs and component weight reduction to be produced.  Applications include oil and gas pipelines, automotive sub-frames, offshore structures and shipbuilding.

These steels are generally manufactured as commercial proprietary steels by speciality steel manufactures to their own codes and internal procedures for specific applications.   The properties of this range of metals are often achieved with a special heat treatment regime by anealing in the intercritical range with controlled quenching to attain the desired properties.

Those steels provide strength-to-weight ratios over conventional low-carbon steels for only a modest price premium.   HSLA steels are available in all standard wrought forms -- sheet, strip, plate, structural shapes, bar-size shapes, and special shapes.

Typically, HSLA steels are low-carbon steels(0,06% to 0,12%) with 0,4% to 2.5% manganese, strengthened by small additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special rolling and cooling techniques.   Improved-formability HSLA steels contain additions such as zirconium, calcium, or rare-earth elements for sulfide-inclusion shape control.   Elements such as copper, silicon,nickel, chromium, and phosphorus can improve atmospheric corrosion resistance of these alloys with e an associated cost penalty.



Reference Standards

BS EN ISO 4957:2000...Tool steels
BS EN ISO 683-17:1999...Heat-treated steels, alloy steels and free-cutting steels. Ball and roller bearing steels
BS EN ISO 15156-2:2003..Petroleum and natural gas industries. Materials for use in H2S-containing environments in oil and gas production. Cracking-resistant carbon and low alloy steels, and the use of cast irons
BS EN 10028-2:2003...Specification for flat products made of steels for pressure purposes. Non-alloy and alloy steels with specified elevated temperature properties
BS EN 10028-4:2003...Specification for flat products made of steels for pressure purposes. Nickel alloy steels with specified low temperature properties
BS EN 10250-3:2000...Open steel die forgings for general engineering purposes. Alloy special steels




Relevant Steel Links
  1. Steel Alloys... Materials Engineer- A short but comprehensive description of steel alloys
  2. Nickel Alloys... AZOM.com article- useful notes on nickel alloys
  3. MIL-HDBK-5H: Metallic Materials and Elements for Aerospace Vehicle Structures ... Very informative on alloys
  4. Alloy and Stainless Steels...Plymouth Tech. Paper - Very informative
  5. Database of Steel and Alloy (Marochnik )...Database of Steels and Alloy (Russian,Ukraine etc)- Very useful for interested users
    European steels and Alloys ...Database of European Steels and Alloy and equivalents - Vast amount of useful information.****

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Last Updated 06/04/2012