MME293: Lecture 16 FERROUS ALLOYS: ( Low and High alloy steels)

Outline of this lecture Plain carbon steels Classification of alloy steels Constructional alloy steel systems

Classification of steel Two main groups : Plain carbon Steel Alloy steel Plain carbon Steel : 1 .Low 2 .Medium & 3 High carbon steel Ni, SI, Al, V ,W etc dissolve in ferrite and Cr, Mo, Ti etc dissolves in carbides

Plain Carbon Steel If the primary alloying constituent is just carbon then that steel is called plain-carbon steel low carbon, medium carbon and high carbon steel Plain carbon steels are among the cheapest metallic materials, but still show considerable versatility which make them the most used alloys by humankind Lower carbon steels show very good ductility and toughness Higher carbon steels can be heat treated to obtain greater strength and hardness Use of plain carbon steels: Strength and other requirements are not too severe Used at ordinary temperature and not highly corrosive atmosphere

Need for Alloy Steels Alloy steels are steels where the property of the steel is predominantly determined by elements other than carbon However in many applications plain-carbon steels are simply not adequate. Its primary drawbacks are – Very high strength/hardness/toughness cannot be achieved (Low hardenability) Unacceptable corrosion even in atmospheric conditions (poor corrosion resistance) Property degradation at high temperatures (low strength at elevated temperature) Most limitations of P. C steels can be overcome by alloying.

HIGH ALLOY STEEL-TOOL STEELS

Introduction Any steel used as a tool can be technically termed as a tool steel However, the term is restricted to high-quality special steels made to controlled chemical composition and processed to develop properties useful for working and shaping other materials, e.g., cutting tool [Tool steels – work at high speed] dies for casting or forming [Die steels – work at high temperature] gages for dimensional tolerance measurement They have 0.10 – 1.6 % carbon and alloying elements such as Cr, W, Mo and V to obtain high hardness to resist deformation resistance to wear to achieve economic tool life, and dimensional stability

Alloy design, the manufacturing route and quality heat treatment are key factors in order to develop tools or parts with enhanced properties that only tool steels can offer Benefits like durability, strength, corrosion resistance and high- temperature stability are also attractive for other purposes than pure tool applications For this reason, tool steel is a better choice than construction or engineering steel for strategic components in different industries More advanced materials easily result in lower maintenance costs, lighter parts, greater precision and increased reliability

Functions of various alloying elements Chromium (Cr) Increases hardenability In sufficient excess together with carbon, forms Cr23C6 for wear resistance Molybdenum (Mo) and Tungsten (W) In large percentages forms with C to hard carbide, (Mo-W)6C Upon tempering a quenched alloyed austenite, precipitates as fine particles in martensite and resists growth at low red temperatures, causing resistance to tempering influences Vanadium (V) Hardest of all carbides, V4C3 Resist solution in austenite, remains unchanged through heat-treatment cycles

Classification of tool steels Based on quenching medium Water-hardening steels Oil-hardening steels Air-hardening steels Based on application Hot-worked steels Shock-resisting steels High-speed steels Cold-worked steels Based on alloy content Carbon tool steels Low-alloy tool steels Medium-alloy tool steels

Different classes of tool steels Water-hardening tool steels (Group W) Essentially a PC tool steels (0.6-1.4% C) with small additions of Cr and V in high-carbon varieties to improve hardenability and wear resistance 0.60 – 0.75 % C – hammers, concrete breakers, rivet sets where toughness is the primary consideration 0.75 – 0.95 % C – punches, chisels, dies, shear blades where toughness and hardness are equally important 0.95 – 1.40 % C – woodworking tools, drills, taps, reamers, turning tools where wear resistance and retention of cutting edges are important Proper heat treatment (austenitized, brine/water quench, tempered) results a hard tempered martensite structure with undissolved carbides at the surface and a tough core High hardness, best machinability and decarburization rating, low red-hardness Used for low speeds and light cuts

Shock-resisting tool steels (Group S) Used where toughness and the ability to withstand repeated shock are required Low carbon (0.45 - 0.65%) steels with added Si (to strengthen ferrite), Cr (to increase hardenability), W (to increase red-hardness), and Mo (to increase hardenability) Generally air-hardening, but some needed water-quench to develop full hardness

Cold-worked tool steels (Group O, A and D) The most important and commonly used tool steels Oil-hardened low alloy types (Group O) Contain 0.8% Mn and small Cr (1.5-3.5 %), W (2.5%) Good non-deforming properties, inexpensive Some alloys also contain Si to cause graphitization and improve machinability and good resistance to decarburization Medium alloy types (Group A) Contain 1% C, 3% Mn, 5% Cr, 1% Mo Increased air hardening property and hardenability Excellent non-deforming properties, good wear resistance, fair toughness and red-hardness High-carbon, high-chromium types (Grade D) Contain 2.25% C and 12% Cr; may also contain Mo, V and Co Excellent non-deforming properties, good wear resistance

Hot-worked tool steels (Group H) Used where the tool is subjected to excessive heat, e.g., hot forging and extrusion, die casting and plastic moulding Need good red-hardness; elements used are Cr, W and Mo Good toughness (because of low C), fair wear resistance and machinability but poor resistance to decarburisation

Selection of tool steels Selection of a proper tool steel for a particular job is difficult Best approach is to correlate the metallurgical characteristics of tool steels with the requirements of the tool in operation The performance of tool steel is judged on the basis of productivity, ease of fabrication, and cost Cutting tool – high hardness, good heat and wear resistance Shearing tool – high wear resistance and fair toughness Forming tools and dies – high toughness, high strength, and high red-hardness Drawing and extrusion dies – high strength, high wear resistance, high toughness, and high red-hardness Thread rolling dies – high hardness, high wear resistance, and high toughness

Most important selection factors in choosing tool steels Hardness Toughness Wear resistance Red-hardness Other factors to be considered seriously in individual case Permissible amounts of distortion in shape Tolerable limits in surface decarburization Resultant hardenability Resistant to heat checking Heat treating requirements (temperature, atmosphere, equipment) Machinability

Tool failure Faulty tool design Faulty steel Faulty heat treatment Improper grinding Mechanical overload and operational factor

Stainless Steels Iron/Steel does not have good oxidation resistance So need for corrosion and oxidation resistant steel is understandable Stainless steel is the most important commercial high-alloy steel Why does iron corrode? It easily reacts with oxygen (even more so in presence of moisture) to form various oxides or hydroxides These form as films on the surface of the metal, but they are not adherent films - they tend to fall off, which means fresh metal is exposed to the environment again

How can we make steel “stainless”? So if we can produce a stable oxide film on steel’s surface we can make it corrosion resistant! For this we need alloying element that Will preferentially react with environment in place of Fe Will form stable, adherent, self-healing film Dissolves in Fe sufficiently Cr is the most widely used element that fulfills all the requirements Reason for oxide films not being adherent : mismatch of volume

Stainless Steel steels containing at least 12% Cr are SS Resistance to corrosion, under oxidizing conditions Corrosion resisting property is imparted by: Insoluble adhesive film of chromium oxide (Cr2O3) The film is too thin to be visible but impervious to water and air Quickly reforms (self-healing) when scratched or otherwise damaged

Classifications of Stainless Steel (Classified into four types based on microstructure and strengthening mechanisms) Austenitic stainless steels: Austenite is retained in the room temperature by Ni; best corrosion resistance. Ferritic stainless steel: Contains less Ni than austenitic SS; medium corrosion resistance; less expensive Martensitic stainless steels: least corrosion resistance; Precipitation-hardened stainless steels: ferritic, increased resistance to dislocation motion, hence increased strength.

Adding Cr At least 11-13% Cr is required for satisfactory corrosion resistance If carbon content is low then this type of steel shows ferrite in the entire T range upto melting – Called Ferritic Stainless Steel It has good corrosion resistance, formability, toughness but moderate strength Ferritic SS is non-heat treatable because there is no austenite-ferrite transformation!

Ferritic stainless steels (400 Series)

Increasing strength(Martensitic Stainless steel) To make SS heat-treatable carbon content is raised which increases the austenite region Even at high %Cr (which gives corrosion resistance) austenite can be formed Thus it can be quenched and tempered to obtain martensite thereby obtaining high strength and hardness This is called Martensitic Stainless Steel

Hence, martensitic stainless steels contain 11.5 to 18 % Cr 0.1 to 1.0 % C (both low and high carbon) They attain the best corrosion resistance when hardened from the recommended temperature but are not as good as the austenetic or ferritic stainless steel. Used in high quality knives, blades, surgical instruments etc.

Now put in some Nickel Nickel stabilizes austenite Sufficient added Ni can make austenite be retained upto room temperature! Can’t transform to ferrite because of low diffusion Can’t transform to martensite because too much alloying elements prevent it This is Austenitic Stainless Steel Most common composition – 0.1% C, 1% Mn, 18% Cr, 8% Ni. Called 18/8 SS Advantage over ferritic SS Best corrosion resistance Tougher and more ductile More formable (used in deep drawn parts) Nonferromagnetic (use in stents, electron microscopy) Disadvantage – less machinable than ferritic SS and more expensive

Austenitic stainless steels (200 and 300 Series) Most common stainless steel (roughly 70% of total SS production); The structure is austenitic; nickel is a strong austenite stabilizer Contains 0.15% C (max), 16% Cr (min) and Ni or Mn. Must have at least 23% Cr plus Ni Addition of 2-3% Mo enhances corrosion protection in neutral salt solutions Addition of Ti or Nb for welded products by forming TiC or NbC instead of (FeCr)4 C

Not heat treatable, hardenable by solid solution strengthening and cold working Non-magnetic (although cold-worked steels show some degrees of magnetism) Most expensive due to high Cr and Ni content Used for flatware, cookware, architecture, automotive, etc.

Be wary of sensitization! Sometimes when stainless steel experiences heating to a certain T, the grain boundaries of the alloy can corrode preferentially. This state of susceptibility to “intergranular corrosion” is called sensitization. Frequent problem in welding of SS (austenitic stainless steel most susceptible); in that context it is called “weld decay” Mechanism The optimum temperature for precipitation of carbide is ~ 650 C The precipitation of (FeCr)4C at grain boundaries causes the concentration of Cr in the adjacent austenite to fall below that required for corrosion resistance It can be eliminated by: a) lowering carbon to 0.03%, b) use Ti or Nb to remove the carbon as TiC or NbC, without lowering the Cr content of the austenite

Unsensitized vs Sensitized SS

Properties of Stainless Steel Strong reducing conditions cause a susceptibility to attack Cl ion is destructive to Cr stainless steels Other properties of stainless steels: strong, tough, high operating temperatures low thermal conductivity (1/3 that of carbon steels), difficult to machine, more expensive than carbon steel