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

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

3. 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

4. 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

5. 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.

9. HIGH ALLOY STEEL-TOOL STEELS

10. 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

11. 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

12. 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

13. 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

14. Different classes of tool steels Water-hardening tool steels (Group W)
Essentially a PC tool steels ( % 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

15. Shock-resisting tool steels (Group S)
Used where toughness and the ability to withstand repeated shock are required
Low carbon ( %) 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

16. 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 ( %), 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

17. 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

18. 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

19. 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

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

21. 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

22. 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

23. 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

24. 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.

25. 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!

26. Ferritic stainless steels (400 Series)

27. 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

28. 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.

29. 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

30. 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

31. 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.

32. 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

33. Unsensitized vs Sensitized SS

34. 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