Strengthening of Steels Presence of interstitial elements such as C - - PowerPoint PPT Presentation

Strengthening of Steels

• Presence of interstitial elements such as C and

N.

• Substitutional Elements like Mo, Nb
• Heat Treatment

– Annealing – Normalising – Quenching – Tempering

• Grain Refinement
• Cold working

Temperature Time Transformation Diagrams

TTT Diagrams – finer details

Example of 0.2% C Steel on Cooling

Lower Bainitic Structure

Lathe martensite formed in 0,08°C steel quenched in brine from 100°C (x20000), Twinned martensite in Fe30%Ni (x110000)

C % 0,02 0,2 0,4 0,8 1,2 Ms °C 520 490 420 250 150

Ms in °C = 561 - 474 (% C) - 33 (% Mn) - 17 (% Ni) - 17(% Cr) - 21 (% Mo).

Mechanical Properties of Normalized and Annealed Steels

Limit of Steels at High Temperatures

• Mainly due to High Temperature Strength

– Ferritic Structure – No strong Strengthening Mechanisms

Creep Strength as a function of temperature for various steels Temperature oC

Creep Strength of some Low alloy steels vs 304 SS

Ratios of RT tensile and Y.S of 21/4 Cr -1Mo steel as a function of Temperature UTS YS

HSLA

• High Strength Low Alloy Steels
• Mainly possess adequate strength and corrosion

resistance

• Main Alloying elements are Cr, Mo and Nb
• Important Alloys are :

– 21/4Cr-1Mo – economizer – 9Cr-1Mo – super heater

• Modified 9Cr-1Mo-W and Nb for super critical power

palnts for Tempertaure above 540 to 600oC and Pressure about 100 bar.

Mechanism of Strength and Oxidation Resistance

Strength comes from ISS – C SSS from Mo Secondary Carbides MoC2 and lower Chromium carbide Corrosion resistance comes from Cr For 21/4Cr steels, the oxide scale is iron rich Cr spinel oxides and outer Fe2O3. For 9Cr steel, the scale is Cr-rich spinel tending toward chromium oxide

Heat Treatment

The steels are used mostly in Normalized and tempered conditions. Normalization can form nuclei of carbide ppt, which on tempering result in uniform distribution and size which imparts strength. However time and temperature is very

• important. It is important not to form bulky

carbides such as M23C or M7C

Tempering of 21/4Cr-1Mo steels

21/4 Cr-1Mo-V Steel

Micro Alloy Steels

• Steels with 0.05 to 0.10% of elements

such as Nb, Mo,Ti, W, Ta, V and/or B provide very high strength to the steel, with total concentration not exceeding 0.15%.

• Use of Micro-alloy Steel in car bodies has

helped in reducing the thickness of plate and thus has made them lighter.

• The Yield Strength is [of the order of 500-

750 MPa.

Example of Micro alloyed Steels

Corrosion Resistance

• At room temperatures

– Organic paint Coatings – Galvanization – Electroplating

• High Temperatures

– Alloying – Mainly Cr – High Temperature coatings ?

Corrosion Resistance of Steels at Moderate Temperature applications

• Usually steels are protected by modification of

environment – by adding inhibitor Chemicals.

• Ion exchange, adding of oxygen scavengers,

phosphate treatment to control hardness and TDS.

• Heat exchanger tubes are subjected to Hot

conditioning – which is exposing them to steam for 24-48h to develop a magnetite layer.

• Fire side corrosion problems are taken care by

cladding or coatings with Ni-Cr alloys to save it from sulphidation and hot corrosion.