MSE-226 Engineering

Materials

Lecture-7

‘’ALLOY STEELS’’

‘’Tool Steels’’

FERROUS ALLOYS

Plain Carbon Steels Alloy Steels Cast Irons

- Low carbon Steel

- Medium carbon steel

- High carbon steel

- Low alloy steels

- High alloy steels

- Stainless steels

- Grey irons

- White irons

- Malleable irons

- Nodular irons

TYPES of FERROUS ALLOYS

Carbon steels are regarded as steels containing not more than 1.65% Mn,

0.6% Si and 0.6% Cu, all other steels are regarded as alloy steels.

Purpose of alloying

1) Increase hardenability

2) Improve mechanical properties at either high or low temperatures

3) Improve toughness at any minimum hardness or strength

4) Increase wear resistance

5) Increase corrosion resistance

6) Improve magnetic properties

ALLOY STEELS

ALLOYING ELEMENTS

Group 1 Group 2

Elements dissolve in ferrite Elements which combine with carbon

to form simple and complex carbides

ALLOY STEELS

ALLOYING ELEMENT GROUP1

Dissolved in ferrite

GROUP2

Combined in carbide

Nickel

Silicon

Aluminum

Copper

Manganese

Chromium

Tungsten

Molybdenum

Vanadium

Titanium

Ni

Si

Al

Cu

Mn

Cr

W

Mo

V

Ti

Mn

Cr

W

Mo

V

Ti

Ni, Al, Si, Cu and Co are all found largely dissolved in ferrite.

In addition, in the absence of carbon Group2 elements will also be found

dissolved in ferrite.

ALLOYING ELEMENTS: Group 1

The effectiveness of these alloying

elements in strengthening iron

The hardening effect of the dissolved elements is small and they have

very little effect on strengthening the steel.

Group 1: Elements dissolve in ferrite

2) Carbide forming alloying elements; (Mn, Cr, W, Mo, V, Ti)

Carbides found in steel are hard and brittle, their effect on the room temperature

tensile properties is similar regardless of the specific composition.

The presence of elements that form carbides influences;

1) Hardening temperature

2) Soaking time

3) Hardenability

* Cr ; Cr7C3 , Cr23C6

* Mo ; Mo2C

* V ; V4C3, VC

* W ; WC W2C

* Ti ; TiC

ALLOYING ELEMENTS: Group 2

Following carbides may occur;

Tool steel : High quality special steels used for cutting or forming purposes

both in hot and cold condition.

The carbon content is between 0.1-1.6% and they also contain alloying

elements like Cr, Mo and V.

Tool steels offer better durability, strength, corrosion resistance and thermal

stability.

They are used in applications such as blanking, die forging, forming, extrusion

and plastic forming

ALLOY STEELS : TOOL STEELS

CLASSIFICATION OF TOOL STEELS

WATER HARDENING STEELS

Tool Steel Type Prefix Specific Types

COLD WORK W = Water Hardening

O = Oil Hardening

A = Medium alloy Air Hardening

D = High Carbon, High Chromium

W1, W2, W5

O1, O2, O6, O7

A2, A4, A6, A7, A8, A9, A10, A11

D2, D3, D4, D5, D7

SHOCK RESISTING S S1, S2, S4, S5, S6, S7

HOT WORK H H10-H19 Chromium types

H20-H39 Tungsten types

H40-H59 Molybdenum types

HIGH SPEED M

T

Molybdenum types

(M1, M2, M3-1, M3-2, M4, M6, M7, M10,

M33, M34, M36, M41, M42, M46,

M50

Tungsten types (T1, T4, T5, T6,

T8,T15)

MOLD STEELS P P6, P20, P21

SPECIAL PURPOSE L and F series L2, L6

CLASSIFICATION OF TOOL STEELS

Most tool steels are used in the following applications:

Cutting,

Shearing,

Forming,

Drawing,

Extrusion,

Rolling

SELECTION OF TOOL STEELS

During selection of tool steels in any applications service requirements for

application should be carefully examined.

1-CUTTING:

Lathe Drills Tap

Service requirements:

Tool must have high hardness, good heat and wear resistance

SELECTION OF TOOL STEELS

Shear blades Automotive parts made by blanking dies

SELECTION OF TOOL STEELS

Service requirements:

Tools have high wear resistance and fair toughness

2-FORMING

Forming is carried out at high or low temperature and done by forging press

Solid metal is forced into tool impression

either hot or cold by using hot forging or cold-

heading die Forging press

Service requirements:

Tools must have high strength, high toughness, and may require high

red hardness (resistance to heat softening)

Piston rod

impression die made

up of tool steel

SELECTION OF TOOL STEELS

3-ROLLING

Service requirements:

Rolling dies must be hard enough to withstand the forces in forming and

must have sufficient wear resistance and toughness to adjust the

stress developed.

SELECTION OF TOOL STEELS

3-DRAWING

tensile force

Ao

Addie

die

Wire drawing Deep drawing

Parts produced by deep drawing: cups,

pans, cylinders and irregular shaped

products

Service requirements:

Drawing dies require high strength and high wear resistance

SELECTION OF TOOL STEELS

Dies made up

of tool steel

Dies used for wire drawing

4-EXTRUSION

Hot extrusion Service requirements:

Cold extrusion dies require toughness to withsatnd outward

pressures and wear resistance.

Hot extrusion dies must additionally posses high red-hardness.

Done at high or low temperature by forcing material into a die

Commonly extruded materials include metals, polymers, ceramics and concrete

SELECTION OF TOOL STEELS

Work piece

Die (tool

steel)

ram

1-Depth of Hardening:

Hardenability increases with alloy content.

Shallow hardening steels;

Group W, carburising grades of Group P, Group F

2-Toughness:

Energy absorbed by a material up to fracture point.

High toughness : S and H groups

Low toughness: cold work tool steels

Hardness

Toughness

Wear resistance

Red Hardness

IMPORTANT SELECTION FACTORS FOR TOOL STEELS

3-Wear resistance:

Resistance to abrasion or resistance to loss of dimensional changes

In general a correlation exists between the hard, undissolved carbide particles

and wear resistance.

4-Red-hardness (Hot hardness):

Defined as resistance of the steel to the softening effect of heat.

Steels that have high red-hardness contain W, Cr and Mo due to formation of

stable carbides.

IMPORTANT SELECTION FACTORS FOR TOOL STEELS

These are essentially plain carbon steels. Some high carbon grades contain

small amount of Cr, V to improve hardenability and wear resistance

Group1: 0.6-0.75%C- High toughness. e.g. Hammers, concrete breakers, rivet sets

Group2: 0.75-0.95%C – High toughness, hardness. e.g. Punches, dies, shear

blades

Group3: 0.95-1.40%C – Increased wear resistance. e.g. Drills, turning tools

WATER HARDENING TOOL STEELS

Concrete breaker Shear blades Punches Turning tools

Most important group of tool steels (used in majority of tool applications)

Types: O-type, A-type, D-type

COLD WORK TOOL STEELS

Group-O (oil hardening):

- Contain Mn (~1%), and smaller amounts of Cr and W.

- Relatively inexpensive

- adequate wear resistance, , fair toughness and red-hardness

Group-A (air hardening):

- Medium alloy type (~1%C, up to 3% Mn, up to 5% Cr, ~1% Mo)

- good wear resistance, fair toughness and red-hardness

Group-D

- High carbon-high alloy types( Up to 2.25%C and 12% Cr + Mo,V,Co)

- excellent wear resistance and nondeforming properties

COLD WORK TOOL STEELS

Applications: blanking and piercing dies, drawing dies for wires, bars, tubes,

taps, forming tools, thread rolling

Blanking die and cut metal by

blanking die Die for wire drawing thread rolling is processes for

forming screw threads

die

In many applications tool is subjected to excessive heat (hot forging, extruding,

die casting and plastic moulding)

Tool steels which have developed for such high temp. applications are called Hot

work tool steels and they have high red hardness

For high red hardness Cr, Mo and W is used

Sum of these alloying elements must be at least 5%.

Basic types;

1- Chromium types

2- Tungsten types

3- Molybdenum types

HOT WORK TOOL STEELS

1) Hot-work Chromium Base (H11-H19)

Composition : Min. 3.25% Cr, Rest: V, W, Mo (carbide former elements)

They are resistant to heat softening. They also have good weldability

Used in extrusion dies, die casting dies, forging dies

2) Hot-work Tungsten Base (H21-H26)

Composition : Min. 9%W and min. 2-12% Cr

They are resistant to heat softening. But they are more susceptable to

brittleness

Used in extrusion dies, madrels

3) Hot-work Molybdenum Base (H41-H43)

Composition : 8% Mo, 4% Cr and small amounts of W and V

More resistance to heat checking than tungsten grades

HOT WORK TOOL STEELS

Casting die for stirring wheel

As a summary, Hot-work tool steels;

have good toughness because of their low carbon content

good to excellent red-hardness

fair wear resistance and machinability

HOT WORK TOOL STEELS

Hot forging die for connecting rod

The most highly alloyed tool steels and contain large amounts of W or Mo along

with Cr, V, Co.

Carbon content varies between 0.7-1% C, some contains 1.5%C

They have excellent red hardness, good wear resistance, poor machinability

Two types of HSS: Mo base (group M), Tungsten base (group T)

The presence of hard carbides makes the tool wear-resistant

Applications:

Cutting tools; such as milling cutters, drills, saws, taps

HIGH SPEED TOOL STEELS (HSS)

PRODUCTION OF TOOL STEELS

Tool and die steels are produced in

Electric arc furnaces (EAF) in small

amounts and then cast into Ingots and

Billets.

Largest amount: 500 kgs

Most common : 200-250 kgs (HSS)

During casting at eutectic point; eutectic carbides (M6C, M4C3, M7C3, M23C6) form

Alloy

carbides

PRODUCTION OF TOOL STEELS

After production tool steels are hot forged to obtain homogenous distribution of

alloy carbides

HOT FORGING

To break down carbides

There are two advantages in forged microstructures;

1) Uniformity

2) Solutionizing becomes easier

1) Heating: During heating much damage may be done to the steel on heating

as cooling

heat slowly or preheat at a lower temperature (to prevent large temperature

gradients)

Overheating should be prevented to overcome grain growth problem. Also,

quenching from excessive temperatures may result in cracking

2) Atmosphere: Surface should be protected against scaling and

decarburization. Inert atmospheres may be used.

3) Quenching media: Water, brine, oil and air

Carbon and low-alloy steels are quenched in brine and water.

High alloy tool steels are quenched in oil, air or molten salts

Sometimes to prevent cracking and distortion interrupted quenching is

applied. Steel is quenched in a liquid bath of salt, then cooled in air

HEAT TREATMENT OF TOOL STEELS

4) Tempering:

They should be tempered immediately just after quenching and before they

have cooled to room temperature to minimize the danger of cracking due to

strains introduced by cracking.

Generally, double tempering is applied to high speed tools

HEAT TREATMENT OF TOOL STEELS

Heating: The parts must be heated slowly or

preheated at a lower temperature (to prevent large

temperature gradients)

time

Temp. (oC)

1300

Austenitization

I.Preheating: Just below critical temp.;

Aim: To equalize the temp. İnside and

outside of the component

Critical temp. Above this temperature crystal

contraction (bcc to fcc)

II.Preheating: Partial dissolution of alloy

carbides( Least stable carbide, Cr, will dissolve),

T= 1050-1100 oC

III.Preheating: If the component is large

T= 1150-1200 oC

Oil hardening

tool steels

HSS

Air cooling

Austenitization period is extremely important. So

to limit or to prevent austenite grain coarsening

small percentages of undissolved alloy carbides

should be left in microstructure

When the steel is heated for hardening, the basic

idea is to dissolve the carbides to such a degree

that the matrix acquires an alloying content that

gives the hardening effect—without becoming

coarse grained and brittle.

HEAT TREATMENT STEPS OF TOOL STEELS

1) Complex carbides don’t dissolve even at high temperatures. This serves to

lower the carbon and alloy content of austenite. Higher temperatures and soaking

times are required for dissolution of alloy carbides.

2) Undissolved carbides also reduce the grain growth.

Both these effects reduce the hardenability of steel

HEAT TREATMENT STEPS OF TOOL STEELS

COOLING: Tool steels may be hardened by quenching in oil or cooling in air.

1300oC

Oil hardened tool

steels

HSS

During cooling;

1) Contraction due to cooling

2) 4% expansion due to martensite

formation

QUENCH CRACKS occur

(solution: use step quenching)

STEP QUENCHING

To equalize the temperature inside and outside of the component, parts

are quenched to T>Ms and wait for long time then cooled in air.

As quenched tool or die

1)Undissolved alloy carbides (If this is HSS: M6C; If D1 or D2 : Cr-carbides)

2) Martensite

3) Retained Austenite (quenching medium T (R.T) > Mf )

HEAT TREATMENT STEPS OF TOOL STEELS

TEMPERING:Since high wear resistance is required temper the steel at around

550oC

Tempering temp.

HRc

As-quenched

hardness

Secondary hardening (alloy carbides)

~550oC

At 550oC;

1) Some alloy carbides precipitate

2) Martensite becomes tempered martensite

3) Additionally, there is retained austenite in the structure

HEAT TREATMENT STEPS OF TOOL STEELS

pearlite

bainite

550oC

T(oC)

Log t

2 hrs of tempering

‘CONDITIONING’

Example: 12% retained-

84% Martensite

4% Undissolved carbides

1.5% retained-

10.5% fresh martensite

84% Temp. Martensite

4% Undissolved carbides

Tempering +

Quench

I. Tempering

II. Tempering (for tempering 10.5% fresh martensite)

III. Tempering (sometimes necessary) MULTIPLE

TEMPERING

HEAT TREATMENT STEPS OF TOOL STEELS

COOLING AFTER TEMPERING: Subsequent to tempering cooling to room

temperature results in transformation of some austenite to martensite