Metallurgy

Metallurgy plays a vital role in design and fabrication of boiler components. Selection of materials for a particular service, raw material evaluation, thermal and mechanical processing of materials, and welding – all these functions of a boiler fabricating industry are based on the principles of physical metallurgy. Yet most of our young boiler engineers, mostly guided by Code requirements and past experience, manage the affairs with alarmingly little knowledge of metallurgy. It is no wonder, then, to see them miserably fail in making decisions when they encounter a new challenge either in the drawing-board or in the shop-floor.

Typical textbooks on physical metallurgy deal exhaustively with crystal structures and phase diagrams before Fe-C diagram is even introduced. Then these books go on to describe various types of steels and cast irons and a host of non-ferrous materials giving little emphasis on Cr-Mo steels, welding, stress-relieving and other topics in which our boiler engineers are interested.

If a write-up on physical metallurgy which excludes the irrelevant topics in which we have very little or no interest and which includes those topics of practical significance to the boiler industry, is prepared, our boiler engineers can, I feel, quickly grasp and absorb the required concepts of physical metallurgy with ease. This work is an attempt in that direction.

This write-up titled, "Essentials of Physical Metallurgy for Boiler Industry" is just a compilation of those topics in physical metallurgy relevant to the boiler industry, picked up from a few textbooks and handbooks, interwoven with related provisions of Codes and Standards. This work confines itself to a few materials commonly used in boiler fabrication industries viz., carbon steels with C < 0.8 %, Cr-Mo steels with Cr ≤ 2¼ %  &  Mo ≤ 1 % and a few austenitic stainless steels; the concepts presented in this write-up should not be extended to other materials without caution.

     This write-up has been split into four parts and presented below. Click on "Read more" tag to view each part in full.

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Reviewed by Sri.V.Thyagarajan, General Manager, M/s.Bharat Heavy Electricals Limited, Tiruchirappalli:
 "The text has been prepared very well, and meant for a mechanical engineer not conversant with metallurgical jargon. This is a very practical and useful compilation and recommended for reading for all mechanical engineers who require some background of metallurgy for their day to day working."

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Essentials of Physical Metallurgy for Boiler Industry - Part 1/4: An Introduction to Carbon Steels

CHAPTER 1. MECHANICAL PROPERTIES OF STEEL

1.1  Review of Mechanical Properties

The usefulness of engineering materials depends primarily on their mechanical properties such as strength, hardness, and ductility and creep resistance. Most of the mechanical and thermal processes of steel components, which are of metallurgical importance, involve modification of these properties. It is, therefore, instructive to keep in mind the definition and significance of these properties prior to learning the principles of physical metallurgy. A brief definition of important mechanical properties is given below:

1.1.1      Yield Strength: It is the stress at which plastic deformation of the steel starts. For ferritic steels, it is the stress at which the specimen elongates without an increase in load in uniaxial tensile test. For austenitic stainless steels, it is the stress at which a permanent set of 0.2% or a strain of 0.002 mm / mm occurs.

1.1.2      Tensile Strength: The ultimate tensile strength is the maximum stress the steel can withstand before fracture.

1.1.3      Ductility: It is the ability of the steel to deform plastically before fracture, measured by the percentage elongation of the specimen in uniaxial tensile test. Steel is said to fail in a brittle manner when it fractures with little plastic deformation.

1.1.4      Toughness: It is the ability of the steel to absorb energy before fracture. It is measured by the energy absorbed by a notched specimen before its fracture under impact load in impact testing machine.

1.1.5      Hardness: It is the resistance of the steel to indentation under a heavy load.

1.2 Significance of Mechanical Properties

A boiler component like drum, pipe, tube, etc, is said to fail, when a significant plastic deformation or fracture occurs in service. Yield strength, which governs the plastic deformation and tensile strength, which governs the fracture are, therefore, important properties of steel in the design of a boiler component.

Only if steel is ductile enough, forming fabrication operations like bending of pipes and swaging of tubes can be successfully carried out. Parts made of ductile steels show visible warning signs like bulging well before fracture in service.

Boiler parts made of ductile steels may fail in a brittle manner under impact loading or tri-axial state of stress. Impact loading includes mechanical and thermal shocks, i.e., increase in loading or thermal gradients in a relatively short time. Tri-axial state of stress exists in boiler pressure parts of very high thickness like drums and main steam piping. In such environment impact strength of the steel governs the failure of the boiler components.

A relatively high hardness zone of boiler component is normally, but not always, indicative of low ductility.

Material specifications usually stipulate the limiting values of all or some of these properties.

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Essentials of Physical Metallurgy for Boiler Industry - Part 2/4: Fundamentals of Heat Treatment 

CHAPTER 4. HEAT TREATMENT

4.1 Recrystallization

Crystalline grains of solid steel try to reduce their surface energy to go to a more stable form. As a result, they try to reduce their outer surface. This explains their tendency to assume equiaxial shape and to grow.

These fundamental recrystallizing forces, which promote spheroidising and grain growth are present at all temperatures but the forces are greater as the crystals become finer and depart further from spherical shape. Opposed to these recrystallizing forces are strength and rigidity of the material, which will drop as the material is heated. Crystals of a material will begin to merge and grow at a particular temperature, known as recrystallization temperature.

4.2 Annealing

Also known as full annealing, it consists of heating the steel to the austenitising temperature (UCT + 20-30 °C) and soaking it there for a definite time and then cooling it back slowly to room temperature, usually in the furnace.

Let us consider a 0.25% carbon steel valve casting, conforming to the specification SA216 WCA, solidifying in the mould from liquid state. Since it passes through very high temperatures, i.e., 1400-1100ºC for sufficient time, grains get coarsened. The coarse-grained austenite at UCT is transformed into coarse-grained ferrite and pearlite at LCT. Such a coarse grained casting is brittle and weak, which can be, however, made strong and ductile by refining the grains by annealing. See fig.4.4.

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Essentials of Physical Metallurgy for Boiler Industry - Part 3/4 - An Introduction to Alloy Steels & Stainless Steels

CHAPTER 5. Cr-Mo FERRITIC STEELS & Cr-Ni AUSTENITIC STEELS

5.1 Alloy Steels

            The so-called plain carbon steel is apt to be fancy to the extent of containing measurable quantities of at least half a dozen other elements like Mn,Si, etc. each of which has its own individual effects on the properties of the steel. But when alloying elements are added on a specific purpose and in such a quantity that the properties of the steel are significantly altered, then the steel is called alloy steel.

5.2 Ferritic and Austenitic Steels

            Carbon steels exhibit poor creep properties and are, therefore, not normally used at metal temperatures beyond 450°C. Addition of Mo in ½-1% increases the resistance of the steel to deformation at elevated temperatures due to the formation of its carbides. But these carbides are not stable at  elevated temperatures for long time.

            Addition of Cr in 1-2¼% to Mo steels stabilizes these carbides. Cr-Mo steels in various compositions like 1Cr-½Mo, 1¼Cr-½Mo and 2¼Cr-1Mo are found to exhibit good creep properties. These steels are used over a wide range of service metal temperatures i.e., from 450-575°C.

            When Cr is added in excess of 11%, it increases the resistance of the steel to corrosion. Addition of Ni depresses the LCT of the steel from 723°C to lower temperatures – 8% Ni ensures that both LCT & UCT are depressed to temperatures lower than room temperature so that the steel is fully austenitic at room temperature. 18%Cr-8%Ni steels are therefore known as austenitic stainless steels, which also exhibit good creep properties at temperatures as high as 600°C.

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Essentials of Physical Metallurgy for Boiler Industry - Part 4/4: Fundamentals of Welding Metallurgy

 CHAPTER 6. WELDING

            Welding is characterized by a very high peak temperature, above 1500°C, but retained only for a very short time. Therefore, there will be no appreciable grain growth unlike in the case of casting. The plate or pipe parts being joined act like a huge sink of heat resulting in a drastic cooling of fused weld pool of little volume. Such a rapid cooling rate is of the biggest concern during welding because it has a potential of adversely affecting the microstructure of not only the solidified weld metal but also the HAZ of the base metal.

6.1 Microstructures of Weld & HAZ

            The microstructure of the weld metal will be noticeably different from that of the base metal or HAZ because it represents the as-solidified molten metal pool under rapid rate of cooling. See fig. 6.1. Typical microstructure of the weld metal of low carbon and low alloy steels consists of ferrite in different shapes and locations and bainite. They are very fine-grained generally.

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