tool steel_col.pdf

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TOOL STEEL TOOL STEEL

Bibliography: The metallurgy of Tool Steel P.Payson

Alloying Elements

Main alloying elements in the tool steels: C - from 0.05 to 2.35 % - The wear resistance is proportional to the hardness

and therefore to the C percentage; the toughness values have an inverseand therefore to the C percentage; the toughness values have an inverse

proportional rate in respect to hardness.

Mn, Si from 0.15 to 3.0 % - They increase the quenchability ; the Mn

combines with S; the Si > 1% decrease the scale formation.

Ni generally lower than 0.5 %.

Cr (0.2 - 12 %) - Carbide forming element; it increases the quenchability.

V from 0.2 to 5 % - Carbide forming element

W (0.5-20%), Mo (0.15-10%) Carbide forming element, they increase the

Prof. G. Ubertalli 18/01/2011

W (0.5-20%), Mo (0.15-10%) Carbide forming element, they increase the

quenchability.

Co from 5 to 15 % - It induce hot hardness even if it is not a carbide

forming element.

Other alloying elements: Al, Ti, Zr, N, Cu added to deoxidized and to control grains size.

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Types of tool steels

Wear resistance

Tool machining steels

Cold working

Hot working Hot working

High-speed steel

Prof. G. Ubertalli

ABRASION

Abrasion at high strength, or grinding

Scratching of surface in abrading media Scratching of surface in abrading media

Abrasion at low strength, or scraping

Free scraping (i.e. sanding nozlles)

Penetrating or ripping abrasion

Impact (i.e. sanding process)

Cr-Mo eutectoidic steels are adopted.

C=0.6-1.0 Mn=0.8 Si=0.25-0.8 Cr=1-3 Mo=0.5

Prof. G. Ubertalli

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TOOL STEELS

Shearing and stamping Shearing and stamping

Hot working die

Chip working operations

Prof. G. Ubertalli

Classification of tool steels

High-speed - Mo base symbol M (M1,M2,M3,M4,M6,M7,M10, ?)

High-speed - W base symbol T (T1,T2,T4,T5,T6,T8,T9,T10)

Hot work - Cr base symbol H (H10,H11,H12,H13,H14,H15,H16,H19)

Hot work - W base symbol H (H20,H21,H22,H23,H24,H25,H26)

Hot work - Mo base symbol H (H41,H42,H43)

Cold work - Cr base symbol D (D1,D2,D3,D4,D5,D6,D7)

Cold work - air hardening types symbol A (A2,A4A10)

Cold work - oil hardening types symbol O (O1,O2,O6,O7)

Other types with symbols S, P, L, F, W Other types with symbols S, P, L, F, W

M2 - C=0.80; Cr=4.0; V=2.0; W=6.0; Mo=5.0

T4 - C=0.75; Cr=4.0; V=1.0; W=18.0; Co=5.0

H12 - C=0.35; Cr=5.0; V=0.4; W=1.5; Mo=1.5

D2 - C=1.50; Cr=12.0; Mo=1.0


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Heat treatments

Annealing (to prepare steel in fabrication) Annealing (to prepare steel in fabrication)

Usual heat treatment for tool steels consists

in a heating to obtain austenite.

Then a cooling to obtain: Phases that forms at high temperature, in annealing.

Martensite to have hardening.

The tempering of steel.


Austenitizing

The transformation of the phases of

low temperature needs a certain

time in furnace at A + 50. This facttime in furnace at A3 + 50. This fact

is evidenced in the left image,

considering the two continuous

lines. However the complete

carbides dissolution need longer

time and higher temperatures, as

detectable from the dashed lines in

the image.

In fact, the carbide dissolutionIn fact, the carbide dissolution

temperature in tool steels are very

high and their higher stability

requires higher temperature and

longer time for the dissolution.


Effect of temperature-time combinations on

structures in 0.78% carbon steel; original structure

was fine pearlite.

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Austenitizing

Schematic charts showing Schematic charts showing

changes that occur during heating

in (a) most tool steels, (b) high-

speed steels.

AA austenitizing temperature.

AH quenching temperature to

obtain hardening.obtain hardening.


Austenitizing Temperature

The austenitizing temperature The austenitizing temperature

influences the hardness after

quench (continuous lines

) and the austenite

grain size dashed lines (- - - - -).

In some case the hardness has

an asymptotic trend (A) while in

others it reaches a maximum

an then decrease (B).an then decrease (B).

The austenite grain size can

respectively growth, according

the lines C, D or E.


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Temperature

(a) Mainly fine spheroidal

carbide in ferrite.

(b) Residual ferrite and (b) Residual ferrite and

carbides with austenite

(now tempered martensite).

(c) Small amount of residual

ferrite and carbide with

austenite (now tempered

martensite).

(d) Residual carbides in

Type H13 tool steel: 0.35% C; 5.0% Cr; 1.1% V; 1.5% Mo.

Etched in picral-HCl, 1000X.

(d) Residual carbides in

austenite (now untempered

martensite).


The transformation of Austenite

Austenite is denser that any of

its transformation product: its transformation product:

therefore, steel undergoes a

volume expansion whenever

austenite transform.

Such changes is superimposed

on the normal volume changes

caused by heating an cooling,

as shown in figure.

Dilatation curves of 1.0 % C

carbon Type W1 steel.


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Quench

During quenching austenite

transforms in martensite. transforms in martensite.

A percentage of austenite is

untransformed and

remains in the component.

Stabilization of retained austenite

in 1.1% carbon Type W1 steel,

caused by holding at room

temperature.


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Austenitizing

The austenitizing tempe-

rature, also, influences the

martensite formation be-martensite formation be-

cause the temperature

influences the carbon and

the alloying elements

percentage.

Effect of austenitizing


Effect of austenitizing

temperature on formation of

martensite during continuous

cooling in 1.1% carbon,

2.8% chromium steel.

Austenitizing

The austenitizing temperature

influences the austenite influences the austenite

composition, the resulting TTT

curves TTT and the mechanical

and workability properties.

Example of Type L3 steel:

a) Austenitized at 1550 F (845C);

many residual carbides, fine many residual carbides, fine

austenite grain size. (9).

b) Austenitized at 1950 F (1065C);

all carbides dissolved, coarse

austenite grain size (3).


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TTT Curves of Type D2 steel


TTT Curves of Type H12 steel


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TTT Curves of Type H13 steel


Tempering

Effect of carbon Effect of carbon

content on hardness

changes during first-

stage tempering in

plain carbon steel.


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The 4 stages of tempering

1) The hardness of the martensite decreases during the first stage of

tempering if the carbon content is above about 0.25%; however, there is

practically no change in hardness during this stage if the martensite is lowpractically no change in hardness during this stage if the martensite is low

in carbon. This drop in hardness is accompanied by a precipitation of a

transition carbide (Fe2,4C and is designed epsilon carbide). As the

tempering temperature approach about 400 F (245 C) the epsilon carbide

goes back into solution, and the stable iron carbide, Fe3C, cementite,

begins to separate from the decomposing martensite. The alloying

elements (except C and N) have no effect on first-stage tempering.

2) Transformation of retained austenite. The alloying elements affect this

stage indirectly, first, in the effects they have on the presence of retainedstage indirectly, first, in the effects they have on the presence of retained

austenite, in the hardened steel, second, in the effect they have on the

transformation of austenite at relatively low temperature. Also the

presence of martensite influences this transformation. The austenite

transform in bainite in a percentage that depends on the TTT curves.


I 4 stadi del rinvenimento

3) The third stage of tempering consists of the precipitation and

growth of cementite, Fe3C, from the residual low-carbon

martensite or the decomposed martensite which existed at the endmartensite or the decomposed martensite which existed at the end

of the first stage. It is during the early stage of cementite formation

that the tempered steels develops so-called 500 F embrittlement

or hard martensite embrittlement. In the third stage of tempering

Si evidences a big influence.

4) The fourth stage of tempering involves the formation and

precipitation of alloy carbides when steel containing the active

carbide-forming elements, vanadium tungsten, molybdenum andcarbide-forming elements, vanadium tungsten, molybdenum and

chromium are tempered over the range of about 900 1300 F

(480 700 C). This induces a increase of hardness (red

hardness), very useful in components that work in hot conditions.


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Tempering - Strength

Correlation betweenCorrelation between

tensile properties and

tempering in Type H11

steel.

C = 0.35%

Cr = 5.00%

V = 0.40%


V = 0.40%

Mo = 1.50%

Red hardness

Hardness and

tempering temperature tempering temperature

in steels Mo containing.

The time at tempering

temperature is constant

for all the steels.


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Red hardness

Temperature-time curves

versus hardness of tempered

5.0% molybdenum steel.


(Master Tempering Curve)

Master tempering

curves of 5 hot work

steels. Resistance of steels. Resistance of

the different steels to

softening at elevated

temperature are

readily apparent from

this chart.


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Silicon effect

Effect of silicon to the Effect of silicon to the

resistance to softening of

3% nickel steels - SAE

2340 - over the tempering

range 400-600 F (205-315

C).


Temper brittleness

This becomes manifest when medium-

carbon, low alloy steels are cooled

slowly from the tempering temperature.slowly from the tempering temperature.

If the steel is quenched from the

tempering temperature, the

embrittlement is avoided.

C-notch impact versus tempering

temperature of 4 tool steels. Impact test

made at room temperature.

Steels hardened as follows:Steels hardened as follows:

-Type A2, austenitized at 1450C, air-cooled.

-Type A6, austenitized at 870C, air-cooled.

-Type L6, austenitized at 845C, oil-quenched.

-Type S5, austenitized at 900C, oil-quenched.


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Tempering curve of Type D2 steel

C = 1.50%

Cr = 12.00%Cr = 12.00%

Mo = 1.00%

Effect of AH temperature

on the hardness at different

tempering temperature.


A large amount of retained austenite in the 2050 F sample causes low hardness in

the steel as hardened, and the transformation of this retained austenite, together

with the precipitation of alloy carbide particles, cause the increase in hardness after

the 850 to 1000 F tempers.

Toughness transition temperature

Transition temperature curves of tough and embrittled conditions of

Type P6 steel. Transition temperature (inflection point of curve) is

estimated at about -25C for the tough condition (rapid-cooled) and at

about 135C for the embrittled condition (slow-cooled).


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tool steel_col.pdf