Unit 3 effect of alloying additions on steel

Austenite-forming elements

The elements C, Ni and Mn are the most important ones in this group.

Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature.

An example of this is the so-called Hadfield steel which contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite.

Another example is austenitic stainless steel containing 18% Cr and 8% Ni. The equilibrium diagram for iron-nickel, shows how the range of stability of

austenite increases with increasing Ni-content. An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C.

Australia MangaNiAusteNi te -N neighbour M , Mn

The equilibrium diagram for iron-nickel, shows how the range of stability of austenite increases with increasing Ni-content.

Austenite region

Austenite forming elements

The most important elements in this group are Cr, Si, Mo, W and Al.

The range of stability of ferrite in iron-chromium alloys is shown in Figure .

Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting.

Another instance of ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.

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Ferrite forming elements

Fe-Cr alloys in the solid state containing more than 13% Cr

are ferritic at all temperatures up to incipient melting.

Ferrite forming elements

Carbide-forming elements

Several ferrite formers also function as carbide formers.

The majority of carbide formers are also ferrite formers with respect to Fe.

The affinity of the elements in the line below for carbon increases from left to right.

Cr, W, Mo, V, Ti, Nb, Ta, Zr.

Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC, Mo2C.

War and car forums(tungsten and elements ending with “um” form carbides)

Carbide-forming elements

Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C.

High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated

M6C,M23C6 and MC.

The letter M represents collectively all the metal atoms.

Thus M6C represents Fe4W2C or Fe4Mo2C;

M23C6 represents Cr23C6 and

MC represents VC or V4C3.

Carbide stabilizers

The stability of the carbides is dependent on the presence of other elements in the steel.

How stable the carbides are depends on how the element is partitioned between the cementite and the matrix.

The ratio of the percentage, by weight, of the element contained in each of the two phases is called the partition coefficient K. The following values are given for K:

Al Cu P Si Co Ni W Mo Mn Cr Ti Nb Ta

0 0 0 0 0,2 0,3 2 8 11,4 28 Increasing

Note that Mn,

which by itself is a very weak carbide former,

is a relatively potent carbide stabilizer.

Carbide stabilizers

In practice, Cr is the alloying element most commonly used as a carbide stabilizer.

Malleable cast iron (i.e. white cast iron that is rendered soft by a graphitizing heat treatment called malleablizing) must not contain any Cr.

Steel containing only Si or Ni is susceptible to graphitization, but this is most simply prevented by alloying with Cr.

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Molybdenum, manganese and elements ending with “um” stabilize carbides

Nitride-forming elements

All carbide formers are also nitride formers.

Nitrogen may be introduced into the surface of the steel by nitriding.

By measuring the hardness of various nitrided alloy steels

it is possible to investigate the tendency of the different alloying elements

to form hard nitrides or to increase the hardness of the steelby a mechanism known as precipitation hardening.

All Car all night

Nitride-forming elements

From Figure , it can be seen that very high hardnesses result from alloying a steel with Al or Ti in amounts of about 1,5%. Base composition: 0,25% C, 0,30% Si, 0,70% Mn

On nitriding the base material in Figure, hardness of about 400 HV is obtained .

According to the diagram the hardness is unchanged if the steel is alloyed with Ni

since this element is not a nitride former and hence does not contribute to any hardness increase.

1.5%

Multi-alloyed steels

The great majority of steels contain at least three components.

The constitution of such steels can be deduced from ternary phase diagrams (3 components).

The interpretation of these diagrams is relatively difficult and they are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions only.

Furthermore, since most alloys contain more than three components it is necessary to look for other ways of assessing the effect produced by the alloying elements on the structural transformations occurring during heat treatment.

One approach that is quite good is the use of Schaeffler diagrams

Multi-alloyed steels

Schaeffler diagrams

Here the austenite formers are set out along the ordinate and the ferrite formers along the abscissa.

The original diagram contained only Ni and Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the equivalents of Ni or Cr respectively.

(all elements in %)Y axis ------> Nickel equivalent=Ni + 30 %C + 0.5Mn + 11.5N X axis ------> Chromium equivalent=Cr + Mo + 1.5 Si +2Nb + 3 Ti

The diagram holds good for the rates of cooling which result from welding.

Schaeffler diagrams

Austenite formers

Ferrite formers

Schaeffler diagrams

Consider a 12% Cr steel containing 0,3% C X axis 12 Y axis 30x0.3=9 Looking at (12,9) we get martensitic phaseNote: the 0,3% C gives the steel a nickel equivalent of 9.

Schaeffler diagrams

Consider a steel with 12% Cr, 8% Ni

if it contains 0-0,5% C and 2% Mn. X=12 y=8+ 30x0.5+ 0.5x2 =24 for (12,24) the phase is austenite

If C is 0.1%, Then y= 8+ 30x0.1 + 0.5x2=12 for (12,12) the phase is martensite + austenite

Elementwise effects

Principles to be used

Principle 1 :

The element can form solid solution if the size is nearly equal or very small compared to the size of the iron atom (solid solution rules)

We can deduce the properties considering the element to act like C to strengthen the iron matrix

Principle 2 :

It can form chemical compound if it has chemical affinity to the carbon. (must be far away from C in periodic table.)

We can deduce the properties considering the element to act like iron, responding to the effect of Compounding with C, form hard materials, distributed in iron matrix)

Data on size and distance

Fe 140 picometres group 8 distance 0Mn 140 7 1Si 110 14 6Cr 140 6 2Mo 145 6 2V 135 5 3Ti 140 4 4W 135 6 2C 70 14 6N 65 15 7

Effect of ManganeseFe 140 picometres group 8 distance 0Mn 140 7 1

Manganese, in association with nickel, performs many of the functions attributed to nickel.

By substituting manganese for nickel, and then combining it with nitrogen, strength is also increased.

It will also interact with the sulfur in stainless steel to form manganese sulfites, which increases the strength at high temperatures.

Nickel (Ni) – 1.increases strength, impact strength and toughness2.imparts corrosion resistance in combination with other elements.3.Improves hardenability, ductility and wear resistance

Effect of silicon

Fe 140 picometres group 8 distance 0Si 110 14 6

Silicon (Si) –1.improves strength, 2.Improves elasticity3.Imparts acid resistance and 4.promotes large grain sizes, 5.which cause increasing magnetic permeability.

Effect of Chromium

Fe 140 picometres group 8 distance 0Cr 140 6 2

Chromium (Cr) – 1.improves hardenability, 2.Improves strength 3.improves wear resistance, 4.sharply increases corrosion resistance at high concentrations(> 12%).

Chromium is the element that makes stainless steel stainless.

It is essential in forming the passive film.

Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element by itself can create the properties of stainless steel.

At about 10.5% chromium, a weak film is formed and will provide

mild atmospheric protection.

By increasing the chromium to 17-20%, the stability of the passive film is increased.

Further increases in the chromium content will provide additional protection.

Effect of Chromium

Effect of Molybdenum

Fe 140 picometres group 8 distance 0Mo 145 6 2

Molybdenum, in combination with chromium, is very effective in stabilizing the passive film in the presence of chlorides.

It is effective in preventing crevice or pitting corrosion.

Molybdenum, next to chromium, provides the largest increase in corrosion resistance in stainless steel.

Industries use 316 stainless because it contains 2-3% molybdenum, which gives protection when chlorine is added to the water.

Molybdenum (Mo) –1. increases hardenability and strength particularly at high

temperatures and under dynamic conditions. 2.improves corrosion resistance

Effect of Vanadium

Fe 140 picometres group 8 distance 0V 135 5 3C 70 14 6from Fe (9 from V)

Vanadium (V) – 1.increases Strength, 2.increases Hardness,3.increases creep resistance4.increases impact resistance

due to formation of hard vanadium carbides,

5.limits grain size.

Effect of Titanium

Fe 140 picometres group 8 distance 0Ti 140 4 4C 70 14 6 from Fe (10 from Ti)

Titanium and Miobium are used to reduce the sensitization of stainless steel.

When stainless steel is sensitized, intergranular corrosion can occur.

This is caused by the precipitation of chrome carbides during the cooling phase when parts are welded.

This depletes the weld area of chromium.

Without the chromium, the passive film cannot form.

Titanium and Niobium interact with carbon to form carbides, leaving the chromium in solution so a passive film can form.

Titanium (Ti) – 1.improves strength and 2.improves corrosion resistance.

Effect of Tungsten

Fe 140 picometres group 8 distance 0W 135 6 2C 70 14 8 for W to C (6 for Fe to C)

Tungsten (W) –

1.increases hardness particularly at elevated temperatures due to stable carbides,

2.refines grain size.