AWS WHC1 04-2001 Welding Metallurgy.pdf

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1、AWS Resources for Engineers Welding Metallurgy welding know-how for engineersii 2001 by American Welding Society All rights reserved No portion of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, including mechanical, photocopying, recording, or

2、otherwise, without the prior written permission of the copyright owner. Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal, personal, or educational classroom use only of specific clients, is granted by the American Welding Society (AWS) provi

3、ded the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923; telephone: (978) 750-8400; Internet: . The Welding Handbook is the result of the collective effort of many volunteer technical specialists who provide information to assist with the design and a

4、pplication of welding and allied processes. The information and data presented in the Welding Handbook, and this chapter, are intended for informational purposes only. Reasonable care is exercised in the compilation and publication of the Welding Handbook to ensure the authenticity of the contents.

5、However, no representation is made as to the accuracy, reliability, or completeness of this information, and an independent, substantiating investigation of the information should be undertaken by the user. The information contained in the Welding Handbook shall not be construed as a grant of any ri

6、ght of manufac- ture, sale, use, or reproduction in connection with any method, process, apparatus, product, composition, or sys- tem, which is covered by patent, copyright, or trademark. Also, it shall not be construed as a defense against any liability for such infringement. Whether the use of any

7、 information in the Welding Handbook would result in an infringement of any patent, copyright, or trademark is a determination to be made by the user. Printed in the United States of Americaiii ACKNOWLEDGMENTS This chapter from the Welding Handbook, Ninth Edition, Volumn 1, “Welding Science and Tech

8、nology,” has been selected by the AWS Product Development Committee as a service to industry professionals. The Welding Handbook Committee and the editors recognize the contributions of the volunteers who have cre- ated, developed, and documented the technology of welding and shared it in the past e

9、ditions of the Welding Handbook. The same enthusiasm, dedication, and willingness to share that they made a tradition continue with this ninth edition of the Welding Handbook. The Welding Handbook Committee and the editors extend appreciation to the AWS technical committees who developed the current

10、 consensus standards that pertain to this volume. They are also grateful to L. P. Connor, editor of Volume 1, eighth edition, and the members of the AWS technical staff for the engineering assistance they generously contributed.iv CONTRIBUTORS WELDING HANDBOOK COMMITTEE H. R. Castner, Chair Edison W

11、elding Institute B. J. Bastian, First Vice-Chair Benmar Associates R. S. Funderburk The Lincoln Electric Company J. M. Gerken, Sr. Consultant I. D. Harris Edison Welding Institute L. C. Heckendorn Intech R Dark Areas, Pearlite), 100X Magnification (before Reduction) and (B) Fine-Grain Aluminum Silic

12、on Alloy Sample with Small Pearlite Patches, 100X Magnification (before Reduction)WELDING METALLURGY 5 The overall arrangement of grains, grain boundaries, and phases occurring in a metal alloy is referred to as the microstructure of the alloy. The microstructure, which is largely responsible for th

13、e physical and mechanical properties of the metal, is affected by the metals chemical composition, thermal treatment, and mechanical history. The thermal and mechanical effects of welding can alter the microstructure, but the changes are confined to the region of the base metal close to the weld. Th

14、e metallurgical changes that occur in these regions, known as the weld metal and the heat-affected zone (HAZ), can have a profound effect on the service performance of a weldment. Many unique phenomena that affect the mechanical properties of an alloy at both low and high tempera- tures occur at the

15、 grain boundaries, where the arrange- ment of atoms is irregular. Because many vacancies, missing atoms, and other defects are found at the grain boundaries, the spaces between the atoms may be larger than normal, permitting individual atoms to move about with relative ease. Thus, the diffusion of e

16、lements (i.e., the movement of individual atoms) through the solvent structure generally occurs more rapidly at the grain boundaries than within the grains. The resulting disarray makes it easier for odd-sized atoms to segregate at the boundaries. This segregation frequently leads to the formation o

17、f undesirable phases that adversely affect the properties of a metal by reducing its ductility, increasing its susceptibility to cracking during welding or heat treatment, or reducing its corrosion resistance. PHASE TRANSFORMATIONS In the field of metallurgy, the term phase transforma- tion (or phas

18、e change) is used to describe the transforma- tion undergone by a material or a distinct portion (phase) of a metal with respect to its crystallographic structure. Critical Temperature Many metals change their crystallographic structure at specific temperatures. The temperature at which one phase is

19、 changed into another phase is known as the critical temperature. For example, at temperatures up to 1670F (910C), the crystalline structure of pure iron is body-centered cubic. From 1670F to 2535F (910C to 1390C), the structure is face-centered cubic, and from 2535F (1390C) to 2795F (1535C), the me

20、lting temperature, it is again body-centered cubic. This type of phase change in a crystalline structure in the solid state is known as an allotropic transformation. Other metals that undergo allotropic transformations include uranium, hafnium, titanium, zirconium, and cobalt. Chemical composition,

21、the cooling rate, and the pres- ence of stress influence the temperature at which the transformation takes place. Metals also undergo a phase change when they melt or solidify. Pure metals melt and solidify at a single tem- perature. Alloys, on the other hand, usually melt and solidify over a range

22、of temperatures. An exception to this rule is the eutectic composition of certain alloys, which is discussed below. Phase Diagram Metallurgical events such as phase changes and solidification are best illustrated by means of a phase diagram, which is sometimes referred to as an equilib- rium or a co

23、nstitution diagram. This graphical repre- sentation plots the stable phases for temperature versus composition for a metal at equilibrium. In metallurgy, the term equilibrium is used to refer to a condition of chemical, physical, thermal, mechanical or atomic bal- ance for a given environment. The e

24、xamination of a phase diagram of a given alloy system permits the deter- mination of the phases present and the percentages of each phase for various alloy compositions at specified temperatures. Phase diagrams also furnish information regarding melting points, solubility, solidification, and the ph

25、ase changes that tend to occur with a change in composition or temperature, or both. Phase diagrams are also an important tool in the field of the metallogra- phy of welding as they provide information about the microstructure of weldments. As most published phase diagrams are based on two- componen

26、t systems at equilibrium, they provide only an approximate description of commercial alloys, which have more than two components and reach equi- librium conditions only at high temperatures. Phase diagrams can be constructed for metal systems having more than two components, but these diagrams are c

27、omplex and difficult to interpret. Nevertheless, phase diagrams are the best technique for studying most alloy systems. A very simple phase diagram for the copper-nickel alloy system is shown in Figure 5. This is a binary sys- tem in which both elements are completely soluble in each other in all pr

28、oportions at all temperatures in both the liquid and the solid states. As can be observed in this figure, phase diagrams are conventionally drawn with the alloy content plotted on the horizontal axis and temperature on the vertical axis. The extreme left- hand edge of Figure 5 represents 100% (pure)

29、 nickel (Ni), while the extreme right-hand edge represents 100% (pure) copper (Cu). At temperatures above Curve A, termed the liquidus (the line on a phase diagram that indicates the tempera- ture at which components begin solidifying during cool- ing or finish melting during heating), the only phase present is liquid metal. At temperatures below Curve B, called the solidus (the line on a phase diagram that indicates the temperature at which components finish