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

essentials

of

PHYSICAL METALLURGY

for

boiler industry

--- for free internal circulation only ---

S.Ganesan, M.Tech.

About the Author

            The author is a graduate in Mechanical Engineering from College of Engineering, Guindy, Anna University, Madras and a postgraduate in Mechanical Engineering from Indian Institute of Technology, Madras. He has fifteen-years experience as an inspection professional in boiler-fabrication industry, having wide exposure to the inspection during fabrication of boiler components in and around Tiruchirappalli, South India in major boiler-fabrication industries like Bharat Heavy Electricals Ltd. and Cethar Vessels Ltd.

PREFACE

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 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.

Any suggestion to improve the usability of this write-up, including critical review, is welcome.

                                                                                   

                                                                       

NOTE:            This write-up was originally written in the year 2001 and is reprinted now in the year 2012 without any change except for a few cosmetic modifications. Therefore, readers are requested to refer to the latest updates wherever Code provisions are referred to in this write-up.

ACKNOWLEDGEMENT

            The author is grateful to the authors and publishers of various handbooks, text-books and codes listed in the reference for the tables and figures used in this write-up. Let the author use this opportunity to thank the executives of Quality Department of BHEL, Trichy for permitting him to use the resources of Shops, Labs and Library of BHEL, Trichy for the preparation of this write-up.
           The author is grateful to Sri.V.Thyagarajan, General Manager (Welding Research Institute and Labs), BHEL, Trichy, who took pains to review the write-up for its technical veracity and suggest certain corrections / additions to the text.

REVIEW BY SRI.V.THYAGARAJAN, GM/BHEL

          The text has been prepared very well, and meant for a mechanical engineer not conversant with metallurgical jargons. 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.

The following are some of my views and can be considered by the author for incorporation in the text.

1. First two paragraphs in 5.2:

C steels have poor creep properties at temperatures beyond 450 deg C and Cementite (Iron carbide - Fe ₃ C) is not stable at temperatures beyond about 425 deg C and has a tendency to break to Fe and C and the phenomenon is called graphitization. Addition of even small amounts of Mo increases creep strength of C steels and also stabilizes the carbide to temperatures up to about 465 deg C. Hence ASME code restricts use of C and C ½ Mo steels to temperatures less than 425 and 465 deg C. Addition of ½% Cr stabilizes the Carbide further and Cr Mo steels are not subject to graphitization. Cr in addition confers oxidation resistance to the steel.

Cr Mo (up to 2 ¼ Cr 1 Mo) steels in various compositions ---- i.e. from 450 – 575 deg C.

2. We can add the following paragraph in 5.4.1 as below.

Sensitization is not normally a problem in boiler service, since SS grades are used only in steam circuits and steam does not cause inter-granular corrosion. Besides the service temperatures of super-heater and re-heater where SS grades are used are in the sensitization range only. ASME does not permit use of austenitic SS in water touched areas.

However sensitization can create a problem during storage/ shut down of the boiler components. This problem is more with un-stabilized grades like 304H and it is preferable to use 347H in such cases. Care during fabrication, storage, erection and proper lay up during outage are required if un-stabilized grades are employed.

3. Third Paragraph in 7.3.1 on Cold Forming Versus Hot Forming. Para ‘If hot forming ….for Cr Mo steel is desirable’, the following can be substituted.

If the heating temperature for hot forming is above 980 deg C, there would be grain coarsening and hence a re-normalizing / re-normalizing and tempering would be required. Similarly if the finishing temperature is below 620 deg C the grains would retain the cold work strain and hence a SR cycle for re-crystallization would be recommended.

4. We can add this at the end of 7.3.3 on tube bending.

Cold worked grains in austenitic SS can affect the creep properties as well as create residual stresses that can lead to stress corrosion cracking if the medium in contact is corrosive. For steam touched service as in the case of Super-Heaters and Re-Heaters corrosion is not a problem, hence ASME gives recommendations on tolerable % cold work strain and hence the R/D ratio of the bend beyond which the bend needs to be solution annealed. For example for design temperatures below 1075 deg F for 304H and 1000 deg F no solution heat-treatment is required for all R/D. Beyond this restrictions on strain in terms of R/D and design temperatures are listed for exemption / performing a solution heat-treatment.

--- end of Review by Sri.V.Thyagarajan, GM/BHEL ---

ABBREVIATIONS

ANSI B31.1           Reference #3

ASME Sec I           Reference #2

BS 1113                 Reference #4

DMW                     Dissimilar Metal Weld

HAZ                      Heat Affected Zone

IBR                        Reference #1

LCT                       Lower Critical Temperature

PFHT                     Post Forming Heat Treatment

PWHT                    Post Welding Heat Treatment

SAW                      Submerged Arc Welding

SMAW                  Shielded Metal Arc Welding

SR                          Stress Relieving

UCT                       Upper Critical Temperature

WPS                       Welding Procedure Specification

REFERENCES

Codes

1.    Indian Boiler Regulations

2.    ASME Boiler and Pressure Vessel Code – Section I

3.    ANSI / ASME Code for Pressure Piping B31.1 Power Piping

4.    BS1113 – Specification for Design and Manufacture of water-tube steam generating plant ( Note:      It has been superseded by BS EN 12952 – Water-tube boilers and auxiliary installations.)

Hand Books

5.     ASM Handbook  Vol.4 – Heat Treating

6.     ASM Handbook  Vol.9 – Metallography & Microstructures

7.     ASM Specialty  Handbook – Carbon and Alloy steels

8.     ASM Specialty  Handbook – Stainless steels

9.     Handbook of  Stainless steels – McGraw-Hill

10.  AWS Welding Handbook  Vol.1 -  Welding Technology

11.  AWS Welding Handbook  Vol.4 -  Materials and Applications

Text Books

12.  Fundamentals of Ferrous Metallurgy – A Allen Bates – ASM

13.  Introduction to Physical Metallurgy – Sidney H Avner - McGraw-Hill

14.  Metallurgical Failures in fossil fired boilers – David N French – Wiley

15. Welding Metallurgy – George E Linnert - AWS

 

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.

1.2  Temperature and Mechanical Properties

As the temperature of steel increases, its yield strength and tensile strength drop. Above about 450°C, steel encounters another phenomenon, called creep. Creep is time-dependant plastic deformation at constant stress, much less than the yield strength, at high temperatures. At elevated temperatures, plastic deformation and fracture are governed by creep strength and rupture strength of the steel respectively. Creep strength is the average stress to produce an elongation of 1% in 1,00,000 hours at a particular elevated temperature. Rupture strength is the average stress to produce rupture in 1,00,000 hours at a particular elevated temperature.

1.3  Grain Size and Mechanical Properties

Liquid steel solidifies in units of grains, as observed under optical microscope with magnification of 100X. A grain is a group of crystals of atoms arranged in a specific orientation. Each grain is separated from adjacent grains by grain boundaries. Grain size can vary greatly, depending on the treatment of steel. The ASTM grain size number, N is defined such that            n = 2^N-1, where ‘n’ = No. of grains per square inch when viewed at 100X. The usual range is from 1 to 10. For example, if 32 grains per square inch are observed, grain size is 6 and if 128, grain size is 8.

Plastic deformation and subsequent fracture occurs in steel mainly through the mechanism of slip through crystal slip planes. Grain boundaries offer much resistance to such slips by changing the direction of slip planes. Fine grains contain more grain boundary surface and resist the slip more and therefore, fine grain steels are more ductile and strong than the coarse grained ones. It is also observed that fine grained steels exhibit better impact strength as compared to coarse grained steels.

CHAPTER 2. STEEL MAKING

Steel is essentially iron with its C content limited to less than 2%. It is obtained by oxidizing the pig iron, which, in turn, is obtained by reducing the iron ore. During the reduction-oxidation sequence, certain other elements like Si, Mn, P and S get inevitably dissolved in steel, influencing its properties. An elementary knowledge of this reduction-oxidation-solidification sequence is essential to understand the metallurgical behaviour of steel.

2.1 Extraction of Iron from its ore

            Iron is extracted from its ore, hematite, Fe₂O₃, by reducing it with coke in blast furnace. Limestone is added as flux.

                        Fe₂O₃  + 3C  ----->   2Fe + 3CO

Fe₂O₃  + 3CO  ----->  2Fe + 3CO₂

            The ore contains sand (SiO₂) and phosphates and coke contains S. During this reduction process, following inevitable events occur, which cannot be controlled by the blast furnace operator.

Ø  3-4% of C from coke dissolves in Fe.

Ø  SiO₂ is, to some extent, reduced to Si, which goes into Fe.

Ø  Phosphates are, to some extent, reduced to P, which goes into Fe.

Ø  Some S gets dissolved in Fe.

2.2 Production of steel from pig iron

            The iron ore from blast furnace containing 3-4% C is known as the pig iron. Steel is produced by oxidizing and removing significant portion of C from pig iron. Iron ore, air and O₂ are some of the oxidizing agents used.

                        2Fe + O₂  -----> 2 FeO

                        FeO + C   ----->  Fe + CO

            C is reduced to below 2%, S and P are not entirely eliminated but restricted. Most of the ASME carbon steel specifications require a max. C content of 0.3 - 0.35%. But since IBR requires stress relieving during fabrication for steels with C above 0.25 %, Steel makers in India generally restrict C to 0.25%. IBR restricts S and P contents of steel to 0.05% each but most of the ASME specifications are more stringent.

            Some FeO get dissolved in the molten steel. If this dissolved FeO were allowed to remain in the steel, after solidification, the metal would be brittle. To prevent this deoxidation is carried out by adding Si and Mn and sometimes Al, which will reduce FeO to Fe and the resultant oxides like SiO₂ join the slag and get removed from the steel.

2.3 Solidification of liquid steel as ingot

            Steel thus made in the liquid state is poured into moulds to allow it to solidify. The resulting solid mass, called as ingot, is converted into useful product forms like plates, pipes and tubes after being passed through several stages of rolling. During solidification of steel, following events take place.

2.3.1 Segregation: Atoms of Fe, C, Mn, Si, etc., in the liquid steel are very uniformly distributed and in free motion. On solidification these atoms assemble in units of crystalline grains. Crystallization occurs selectively i.e., those elements which lower the melting point of steel (e.g. C) solidifies later. Therefore across the section of ingot there will be variation in chemical analysis. This variation is known as segregation. This is usually carried forward up to the final rolled product.

                For example, for a particular SA515 Gr70 drum plate of 100 mm thick, ladle reported just 0.18% C whereas mid-width mid-thickness product analysis reported as high as 0.28%C.

2.3.2 Volume Shrinkage: As the temperature is reduced, volume shrinks. Voids may be formed inside the ingot. If the void surface gets oxidized by the entrapped air, these voids may not weld together during subsequent rolling of the steel, resulting in defects like laminations in the final rolled product.

2.3.3 Decrease of solubility of dissolved elements: As the temperature drops, dissolved elements in the form of some compounds (e.g. O₂ as CO, FeO, etc.) reach saturation. Coming out, they may make their appearance as gas e.g. CO, or liquids e.g. FeO. Gases may bubble out of the mould, or may be caught in the mass of growing crystals and form holes in the ingot, called blowholes. Liquids may join the slag and get removed from the ingot or be caught somewhere in the mass of crystals and solidify as inclusions.

2.4 Killing of Steel

            Killing of steel refers to the extent of deoxidation of steel, depending on which steel is classified into 3 categories as detailed below. Amount of Si present in the steel is a good indicator of the extent of deoxidation.

2.4.1 Fully Killed Steel: It is the steel to which sufficient Si and Mn are added so that essentially no O₂ remains dissolved. Almost no gas (CO etc.) is liberated in the process of solidification and the steel lies very quietly in the mould. Most of the ASME specifications require the steel to be fully killed. At least 0.1% of Si is required to fully kill the steel.

2.4.2 Semi-Killed Steel: When the liquid metal has been only partly deoxidized, an appreciable amount of O₂ remains in it. O₂ comes out of solution as CO bubbles. They may be trapped in the solidifying mass, forming blowholes, which may or may not weld during subsequent rolling, depending on the extent to which their surfaces are oxidized. Therefore, the quality of steel will be poor as compared to the fully killed steel. Si content is in the range of 0.03 - 0.10%

2.4.3 Rimming Steel: It has a high content of O₂ (200-400 ppm), very little C (<0.1%) and almost no Si (<0.03%). It forms a rim of almost pure iron on the surface forming a very ductile and smooth skin. Welding electrode bare wires are made of rimming steel.

CHAPTER 3. IRON - CARBON DIAGRAM

3.1 Phase Diagrams

            A phase is a physically homogeneous distinct entity. A pure metal exists as solid, liquid, or gaseous phase. In addition, if the metal exhibits allotropy, even in the solid state, the metal can exist in different phases. e.g. Iron exists in liquid phase at temperatures above 1540°C. Between 1540°C and room temperature, it exists in the solid state in 3 different phases depending on the temperature - d-Iron between 1540°C and 1400°C; g-Iron between 1400°C and 910°C; and a-Iron below 910°C.

            Alloys may exist in 3 types of phases in the solid state viz., intermetallic compound, solid solution, or a combination of these two. In intermetallic compound, atoms of constituent elements are arranged in some precise and invariable pattern e.g. Cementite – Fe₃C. In solid solutions, the atoms of constituent elements are arranged in a variable and irregular manner e.g. a crystal made of Cu and Ni may be composed in a range from 7:1 to 1:7.

            A phase diagram of an alloy depicts the phase transformations with respect to temperature for different relative compositions of the constituent elements of the alloy under equilibrium conditions. Equilibrium condition is achieved when cooling is very slow like cooling in furnace with fuel cutoff.

3.2 Iron – Carbon Diagram:

            It is a phase diagram of Fe-C alloy system. Different phases encountered in Fe-C diagram, shown in fig.4.1, are briefly described below:

3.2.1 Ferrite: It is a solid solution of C in a-iron. This is the softest phase of this diagram. Maximum C content in ferrite is only 0.025%. It is stable at room temperature. It etches white under microscope.

3.2.2 Austenite: It is a solid solution of C in g-iron. Maximum content of C in austenite is 2.1%. It is not stable at room temperature and exists only at higher temperatures.

3.2.3 Cementite: It is an intermetallic compound, expressed as Fe₃C. C content is as high as 6.67%. It is very hard and brittle.

3.2.4 Pearlite: It is a lamellar structure of alternate layers of ferrite and cementite. It etches dark under microscope. It is stable at room temperatures.

3.3 Solidification of 0.2% C-Steel

Let us now analyse the changes in the microstructure of 0.2%C steel, as it gets solidified from liquid state. See fig.4.1. At 900°C, above the curve BC, the microstructure entirely consists of austenite. As the temperature drops below the curve BC, say, at 850°C, microstructure consists of austenite and ferrite. The curve BC is known as Upper Critical Temperature line, UCT line. Below 723°C till room temperature microstructure consists of ferrite and pearlite. The line representing temperature 723°C is known as Lower Critical Temperature line, LCT line.

            Solidification phase transformation of 0.2%C steel may be summarized as follows. Above UCT alloy is composed entirely of austenite. Between the UCT and LCT some of the austenite grains get transformed into ferrite. Below LCT, the rest of the austenite grains gets converted into pearlite and the ferrite already formed between UCT and LCT gets carried over.

            Therefore at room temperature the alloy is composed of ferrite and pearlite. As the C content of the steel increases, % of pearlite increases. Thus 0.05% C-steel consists of 97% ferrite and 3% pearlite, whereas 0.35% C-steel consists of 58% ferrite and 42% pearlite.

            All these transformations require diffusion of C atoms across the grain boundaries, as there is large difference in C content of ferrite, austenite and cementite. Diffusion is both temperature and time dependent. Therefore it is assumed that the cooling is carried out very slowly so that at any given temperature, sufficient time is given to the C atoms to diffuse.

            Assume that this sample of 0.2%C-steel at room temperature with a microstructure of ferrite and pearlite is coarse grained. Suppose this sample is reheated slowly. At LCT, pearlite grains get transformed into austenite. At UCT ferrite grains get transformed to austenite, so that the resultant microstructure above UCT is fully austenite. Between LCT and UCT a peculiar phenomenon takes place, i.e., coarser ferrite and pearlite grains get transformed to finer austenite grains so that the resultant austenite grains above UCT is fine grained. When this sample is slowly cooled back to room temperature, fine ferrite and pearlite grains are formed. Thus heating a coarse grained steel from room temperature to UCT and then cooling it back to room temperature results in grain refinement. This is the fundamental principle of annealing and normalizing heat treatment processes.

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