Presentation on theme: "Unit 3 Stainless and tool steels. Stainless steels Used for corrosion and heat resistant applications Contain large amounts of chromium 12% Cr raises."— Presentation transcript:
Unit 3 Stainless and tool steels
Stainless steels Used for corrosion and heat resistant applications Contain large amounts of chromium 12% Cr raises the critical temperatures and reduces the austenite region. With sufficient amounts of carbon, these steels can be heat treated to a martensitic structure. The response to heat teatment depends upon their composition. Indicated by three digit numbers like 304, 402 etc. 2xx-Cr-Ni-Mn- non hardenable-austenitic- non magnetic 3xx- Cr-Ni- non hardenable-austenitic- non magnetic 4xx- Cr- hardenable- martensitic- magnetic 4xx-Cr-non hardenable-ferritic-magnetic 5xx-Cr- low chromium-heat resisting Last two digits are serial numbers to designate a variety of that group.
Martensitic Stainless steels Straight chromium steels with 11.5 to 18% Cr. C 0.15 Mn 1.25 Si 1 For turbine blades and corrosion resistant applications Magnetic Can be machined (poorer machinability than plain carbon steels. Machinability can be improved by adding small amounts of Selenium or Sulphur.) Hot working possible. Can be hardened (by air cooling or oil quenching itself)
Ferritic Stainless steels 14 to 27% Cr Low in carbon but high in Cr compared to martensitic steels. Not hardened by heat treatment. Only moderately hardened by cold working Can be cold or hot worked. Achieves maximum softness in annealed condition. As annealed, their strength is 50% higher than plain carbon steels and corrosion resistance and machinability is better than martensitic steels. Annealing is done to relieve stresses due to welding or cold working. Susceptible to embrittlement during slow cooling during annealing. Since martensite is not formed and since there is embrittlement possibility, these steels are not tempered.
Austenitic Stainless steels Chrome-nickel or chrome -nickel- manganese alloys Austenitic, non magnetic and do not harden by heat treatment. Total content of nickel and chromium is at least 23% Difficult to machine. Can be improved by Selenium of sulfur additions. Best high temperature strength and reistance to scaling. Hence the best corrosion resistance. Cold working causes work hardening. Can be hot worked easily. Type 302 stainless steel is more used.(austenitic) Modified into 22 different alloys. Lowering the carbon to 0.08% gives stainless steel type 304 with improved weldability. Used for most fabrication that needs welding.
Tool steels Steels used to make tools and dies. Tools are used for cutting or forming purposes-to shape other materials. Classified based on 1. Quenching method air hardening, oil hardening, water hardening 2.Carbon content Carbon tool steels, low alloy tool stels or medium alloy tool steels
Tool steels Method followed by AISI (American Iron and Steel Institute) W- Work hardening S- Shock resisting O-Oil hardening (cold work Steel) A-air hardening (cold work Steel) D-High carbon high chromium(cold work Steel) H-Hot work tool steel T-Tungsten base HSS (high speed steel) M-Molybdenum base HSS
Tool steels-properties needed 1.Must be hardenable Definite temperature and definite cooling rates (known to user) for hardening 2.Resistance to decarburisation 3.Resistance to softening on heating (hot hardness) 4.Must withstand high temperatures during operation (heat resistance) 5.Must withstand high wear environment (wear resistance) 6.Must withstand impact load (toughness) 7.Possible to shape (machinability)
Selection of Tool steels The type of operation decides what property is required in the tool. Accordingly, the composition of the tool steel is chosen. Cutting- hardness, heat resistance and wear resistance Lathe cutting tool, milling cutters Shearing-hardness and toughness (resistance to fracture) Punches and shears Forming-high strength, impact strength, hot hardness (not losing its strength at hot condition) Punches and rolls for shaping hot steel sheets Drawing and extrusion-toughness, wear resistance and hot hardness. Wire drawing dies
Some popular tool steels The Joint Industry Conference JIC, USA uses the following symbols T- tungsten based high speed steel M-Molybdenum based HSS D-HCHCr (High Carbon High Chromium) A-Air hardening O-Oil hardening W-Water hardening H-Hot work steel S-Shock resisting steel W8 means a water hardening steel with 0.8%C.
HSS Tungsten base T1 C 0.7, Cr 4, V1, W 18 T4 C 0.75, Cr 4, V1, W 18, Cobalt 5 % T6 C 0.8, Cr 4.5, V1.5, W 20, Cobalt 12 Molybdenum based M1 C 0.8, Cr 4, V1, W 1.5, Mo 8 M6 C 0.8, Cr 4, V1.5, W 4, Mo 5, Co 12 (High speed-more heat-more heat resisting elements) Carbon less than 0.8% eutectoid composition
HCHCr D2 C 1.5, Cr12, Mo 1 D5 C 1.5, Cr12, Mo 1, Co 3 D7C2.35,Cr12,Mo 1, V 4 Note: carbon more than 0.8% hence high in carbon
Air, oil and water hardening Air hardening A2 C 1, Cr5, Mo 1 A9 C 0.5, Cr5, Mo 1.4Ni 1.5, V 1 Note: carbon closer to the eutectoid composition Carbide formers are less than in HSS and HCHCr Oil hardening O1 C 0.9,Mn 1,Cr 0.5,W 0.5 Note the presence of Mn which renders the steel more hardenable Water hardening W2C1.4, V0.25 W5C1.1,Cr 0.5
Precipitation hardening steel 17-7 PH Cr17%, Ni7%, Si 0.4, Mn0.7 and C 0.07 Soultion treated in roll mill and supplied. After forming to required shape, they are aged to attain the required increase in hardness and strength.
Precipitation hardening 17-7PH is solution annealed at 1950 deg F,followed by air cooling. This produces austenite with around 20% delta ferrite. In this condition, the alloy is soft and can be easily formed. In Temper hard (TH) sequence,austenite is conditioned by reheating to 1400 deg F. This will precipitate the chromium carbides, reducing the carbon and chromium content of the austenite.Therefore transformations are possible while further cooling. Cooling is continued till 60 deg F to obtain the necessary amount of martensite. Aging is carried out at 1050deg F.
Maraging steels A series of iron base alloys capable of attaining yield strengths upto 300,000 psi. in combination with excellent fracture toughness. Low carbon (0.03%), % Nickel and other hardening elements. Yield strength: The stress at which a material exhibits a specified deviation from proportionality of stress and strain. Fracture toughness:resistance to crack propagation.
Maraging steels As annealed, these steels are martensitic. They achieve high strength on being aged in the annealed or martensitic condition. This martensite is soft and tough as compared to the hard and brittle martensite formed in conventional low alloy steels. This ductile martensite has a low work hardening rate and so can be cold worked to a high degree.
Maraging steels There are two groups, based on hardening element used. 1.The 18% nickel grades use cobalt molybdenum additions 2.The 20% nickel grades use titanium-aluminium-columbium additions.
Maraging steels 18% Nickel Heated to 1500 deg F, held for 1 hour. Soaked to anneal austenite and dissolve hardening elements Co and Mo. Air cooled to 100 deg F, forms martensite of 28 – 32 RC Reheated to 900 deg F, held for 3 hrs- aged to harden. stress relief also occurs at the same time. Hardness of 52 RC can be achieved hrs 1 hr 300 Deg F Ms RC 52 RC
Maraging steels 18% Nickel The interest is in achieving high strength at room temperature. Simple heat treatment carried out at moderate temperature is enough to achieve good properties. Section size, heating and cooling rates are not important. Very low in carbon content and so no problem of decarburization. Protective atmosphere is not required. Low aging temperatures means less distortion. So,no deformation on hardening and not much machining is required after heat treatment.
Maraging steels Effect of additives on maraging strength development Solution treated 110,000 psi After maraging 300,000 psi Iron nickel martensite 25Rc A weak response to maraging is seen after addition of 7% cobalt. The addition of molybdenum alone gives a slight increase in annealed hardness and good maraging response. When Mo is added in presence of 7% Cobalt, an increase in hardness greater than the combined effect of both the elements is seen. Co 7Co+Mo Mo Co Solution treated Rockwell C 2468 %Mo or %Co
Maraging steels 25%Nickel Largely austenitic after annealing. The conversion to martensite is done by ausaging or cold working. Ausaging-conditioning treatment at 1300 deg F. Reduces the stability of austenite by causing nickel titanium compounds to precipitate from the austenitic solid solution. It raises the Ms temperature so that martensite will start forming at room temperature. Cold worked to 25% to start the transformation to martensite and is completed by refrigeration at minus100 deg F.
Maraging steels 20% Nickel Lower alloy content Freedom from cobalt and molybdenum. Useful in certain environments and applications. Compared to the 25% grade, this does not require a conditioning treatment to become martensite. Ms temperature is above room temperature. Disadvantages: Lower in toughness, resistance to stress corrosion cracking and in dimensional stability during heat treatment.
Maraging steels Applications for maraging steels Hulls for hydrospace vehicles Pressure vehicles Motor cases for missiles Mortar and rifle tubing Hot extrusion dies Low temperature structural parts Cold headed bolts (Complex shapes that need to be strong after shaping)
Solution treatment Heating an alloy to a suitable temperature Holding at that temperature long enough to allow one or more constituents into solid solution Then cooling rapidly to hold the constituents in solution. The alloy is in a supersaturated, unstable state.
Age Hardening Aging: A change in properties in alloys, that occurs slowly at room temperature and more rapidly at high temperatures. Age hardening: Hardening after aging, usually after rapid cooling or cold working.
Cold working and Hot working Cold working: Deforming the metal plastically at a temperature lower than the recrystallisation temperature. Hot working: Deforming a metal at such a temperature and rate that strain hardening does not occur. The low limit of temperature is the recrystallization temperature.
Strain Hardening Strain hardening: An increase in hardness and strength caused by plastic deformation at temperatures lower than the recrystallisation temperature.
Recrystallization Recrystallization temperature: The approximate minimum temperature at which complete recrystallization of a highly cold worked metal occurs within a specified time, usually one hour. Recrystallisation: The formation of new strain free grain structure from that existing in a cold worked metal, usually accomplished by heating.