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.

            HAZ is that portion of the base metal lying next to the fusion line of weld, which had not melted but whose microstructure and hence mechanical properties have been altered by the heat of welding. Near the fusion line the peak temperature of HAZ can reach 1400°C. Grain growth and coarsening occur in this region. See fig. 6.2.

           Because of the relatively high cooling rate and large grain size, acicular, rather than blocky, ferrite is formed at boundaries of large grains of fine-pearlite or bainite. Due to coarsened grains, under very high cooling rate, this region has the potential of getting transformed into martensite.

6.2 Residual Stress

6.2.1 Development

            Let us assume that during welding, a small band of base metal adjacent to fusion zone reach an average temperature of 900°C and the rest of the base metal is at an average temperature which is slightly more than the room temperature. The heated band of base metal adjacent to the weld tries to expand, which is restrained by the adjacent large mass of relatively cold portion of base metal. This leads to a compressive stress induced in the heated band. Since a temperature differential of 100-150°C is sufficient to induce stresses of such a high magnitude as to exceed the yield strength of the steel, the heated band soon starts flowing plastically during welding.

            Now after welding, the heated band begins to cool and shrink, which is again restrained by the adjacent large mass of base metal. This leads to a tensile stress induced in the heated band. The large thermal gradient during the heating-cooling cycle of welding, thus, leads to the development of internal stress, called the residual stress.

            The nature and distribution of the residual stress are complicated and difficult to be evaluated. It exists along, across and through-thickness of the weld and base metal, i.e., in a triaxial state. It is comprised of tensile and compressive stresses in equilibrium. It is generally of the order of yield strength of the material at its peak value.

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.

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.

The casting is reheated to its austenitising temperature. UCT of steel can be computed by assuming the UCT curve to be a straight line: 910 – [(910-723) × 0.25/0.8] ≈ 850ºC. Add 30ºC to get 880ºC as the austenitising temperature. Since diffusion rate is dependent on the surface area of a grain, coarse grains, due to their less surface area, dissolve very slowly. Therefore, austenitising temperature is selected around 900ºC. For the same reason, soaking time at this temperature is also unusually longer – 3 - 6 hours are quite common. Then the casting is slowly cooled through the transformation range between UCT and LCT in the furnace with fuel cut off. Since the microstructure of the casting does not change below LCT, casting is usually taken out of the furnace after 500°C. Thus a strong and ductile casting with fine grained structure at room temperature is obtained by annealing. Annealing also reduces the hardness of the casting.

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.