1、STD-AWS PRGWM-ENGL 1977 E American Welding Society The Practical n I n 07842b5 0519407 87T W Heterence WiUe to I Key Concepts for Weldability STD-AWS PRGWM-ENGL 1999 0784265 0519408 72b THE PRACTICAL REFERENCE GUIDE to WELDING METALLURGY- Key Concepts for Weldability Compiled/edited/written by Ted V
2、. Weber Weber online: http: / O 1999 by the American Welding Society. All rights reserved. Printed in the United States of America. STD*AWS PRGWN-ENGL 1999 0784265 0519410 384 TABLE OF CONTENTS Page No . Introduction 1 Definitions 1 Metal Structures . 3 Metal Forms 5 Diffusion . 8 Solid Solubility .
3、 1 0 Shielding and Purging 13 Phase Transformation . 15 Grain Size 20 Stainless Steels 21 Sensitization of Austenitic Stainless Steels . . Aluminum and its Alloys . 24 Copper and its Alioys . 25 Nickel and its Alloys . 25 Repair Welding 26 Residual Stress . 13 Hardness and Hardenability 15 Effects o
4、f Elements . 20 Refractory Alloys . 25 Summary . 27 Selected References 27 Glossary 28 iii STDoAWS PRGWM-ENGL 1777 Introduction Knowledge of welding metallurgy can be beneficial to almost every aspect of fabrication, inspection, and failure analysis. Too often, problems occur re- peatedly because th
5、e metallurgical aspects are not sufficiently understood (note Figure l), and as the old saying goes, ”When you continue the exact same practices, why should you expect different results?” While the subject of metallurgy, and its subset welding metallurgy, encompasses a very large tech- nical base, t
6、here are several basic issues that can be studied and implemented to aid in avoiding prob- lems associated with fabrication and repair weld- ing. These basic issues will be discussed in simple terms and hopefully with an approach that will en- able a non-metallurgist to grasp and apply them in order
7、 to avoid common welding problems. Since carbon and low-alloy steels are used predom- inantly in many industries, these alloys form the basis for much of this metallurgical review. An un- derstanding of the steel basics can then lead to other alloy groups including austenitic stainless steels, coppe
8、r and aluminum alloys, and the high alloys that include the nickel alloy groups. These families of alloys will also be discussed, but to a much lesser degree. , Definitions A discussion of metals requires the first step to be a review of several basic definitions. Many defini- tions used in this gui
9、de are from Websters. A metal is defined as “Any of a class of chemical elements gen- erally characterized by ductility, malleability, luster, and conductivity of heat and electricity.” Examples of met- als include gold, iron, aluminum, and silver. Metals can be found in their natural elemental stat
10、e, such as the case with gold and silver, or combined with other elements such as oxides, sulfides, sulfates, etc. These combined forms of metals are referred to as “ores,” and the elemental metal must be first ex- tracted, or separated from, the other constituents before combining them in desired a
11、lloy forms. An alloy is defined as “A metal that is a mixture of two or more metals, or of a metal and something else. ” The phrase “something else” in the definition can refer to the combinations of metals with ceramics, called ”cermets,” or various other combinations. Some metal alloys occur natur
12、ally while others are combined in furnaces by intent to develop particu- lar mechanical or physical properties. Examples of very common alloys include carbon steel, a mixture of primarily iron and carbon, and the austenitic stainless steels that are primarily mixtures of iron, chromium, and nickel.
13、The man-made alloys also contain many other elements that may affect their properties; these will be discussed later. Figure 1. Liberty ship failures from the World War II era: massive hull fractures due to a combination of poor-quality steel, less-than-adequate welding procedures, and low temperatu
14、res in the North Sea. AWS Practical Reference Guide 1 In addition to the iron-based, or ferrous alloys, there are nonferrous groups of alloys of aluminum, copper, and nickel, to name a few of the more com- mon ones. Additionally, many other elements have their own specialized alloy groups, such as c
15、obalt, tungsten, and molybdenum. Alloy development usually occurs due to a need for specialized proper- ties not currently available. Thousands of alloys have been developed to date, and it continues at a rapid pace. Metallurgy is defined as “The art or science of separat- ing metalsfrom their ores,
16、 and preparing them for use by smelting and refining.“ This definition includes both “extractive metallurgy,“ the separation of metals from their ores, and “physical metallurgy,“ which prepares them for industrial use. Welding metallurgy is a further extension of physical metallurgy that spe- cifica
17、lly applies to those metallurgical considerations of the welding operation needed to develop an end product that can be used safely and economically. Figure 2 shows modern fabrication shops. Metals are very unique materials permitting our in- dustrial world to design and fabricate many very useful i
18、tems not only for industrial use but for our personal needs as well. Metals vary widely in cost, availability, mechanical and physical properties, heat treatments, corrosion-resistance, weldability, and many other less common attributes. The me- chanical, design, or metallurgical engineer has more t
19、han 20,000 different and unique alloys to se- lect from for a particular application. Too often, the weldability aspects are not given enough consider- ation at the start, and when welding difficulties oc- cur, most individuals involved seem surprised. With the vast number of alloys available, and w
20、ith the majority falling into the category of being readily weldable, one would think that welding problems would disappear. However, the art and science of joining a metal to itself, or joining two dissimilar metals, depends on much more than the initial selection of alloys. Welding a metal success
21、- fully and repeatedly requires a great number of welding variables to be considered. These welding variables include: O O O O O O O O O Base metal chemistry, thickness, and heat treat condition Filler metal chemistry, type, and electrode diameter Welding process (choose from over 35 methods) Flux s
22、ystem (if required) Storage of filler metals and fluxes (heated stor- age, warm, dry, etc.) Cleanliness of base and filler metals Joint geometry (V-groove, U-groove, Square Butt, etc.) Joint accessibility (one or both sides, open root or backup strip) Welding heat input (volts, amps, travel speed) I
23、nterpass temperature limits Interpass cleaning Figure 2. Modern fabrication shops utilizing welding equipment and procedures to avoid fabrication problems. 2 AWS Practical Reference Guide STD-AUS PRGWM-ENGL 1999 = 0784265 O519413 093 Weather conditions (sun, rain, snow) Ambient temperature (January
24、in Alaska or July Humidity (Gulf Coast or Arizona desert) Preheat temperature Cooling rate Shielding gases (if required) Purging gases (if required) Post weld heat treatment (if required) Others The length of the list should not cause dismay. Sel- dom, if ever, do all the listed variables come into
25、play for a particular weld project, but often, several of them must be considered jointly when develop- ing welding procedures. While the study of scien- tific principles can assist in developing a welding procedure, experience is also an important factor in that development. Neither source of infor
26、mation can, or should, be ignored in solving welding prob- lems since both can lead to a solution. The trial and error process played an important role in early welding development, and is still a very useful tool for solving day-to-day welding problems. A phrase that supports this process is ”One t
27、est can be worth a thousand expert opinions.” Often, a welding test spec- imen must be prepared to confirm a welding proce- dure and this approach is still used in most code requirements. The compatibility of the base metal, filler metal, and welding process are still best deter- mined through actua
28、lly joining the metal by weld- ing a specimen and testing the result. in Houston) Metal Structures Metals have been in use for centuries, and the early metal artisans had little or no understanding of the science of metals. However, even without that sci- entific understanding, items made of metals
29、found in archaeological sites confirm that metals have been worked into useful shapes for particular tasks for centuries. As shown in Figure 3, representing the early welding days, most metal working and welding was done by the blacksmith, and it relied heavily on trial and error and past experience
30、s. Knowledge was passed on from generation to gen- eration by word of mouth and on-the-job training. Only more recently, within the past 100 years or so, Figure 3. Early welding was based on trial and error, and most of the time was done in the blacksmiths shop with carefully guarded procedures hand
31、ed down within the family from one generation to the next. have metals been studied in a manner to determine their detailed physical, mechanical, and chemical properties. Initially, the metal workers used the trial-and-error process to develop alloys and the optimum heat treatments for making variou
32、s products. This worked extremely well for many applications, and included the use of several alloys, including bronze and steel. The early artisans were very secretive with their metal-working knowledge in an effort to avoid competition. The early welding equipment was quite unsophisticated as show
33、n in Figure 4. To- day, results of various alloy and welding research studies are published throughout the world. In todays world, once the metals have been ex- tracted and refined by the extractive metallurgists, the physical metallurgists and welding engineers must have an extensive understanding
34、of the met- als physical and chemical properties to best de- velop them for end uses. Instruments to accurately measure alloy chemistry have been developed and their use has dramatically increased our knowledge of the metals chemical properties. We can now also measure the coefficients of thermal ex
35、pansion, heat conductivity, electrical conductivity and all the other physical properties necessary to further our understanding of metals. Mechanical tests have been developed to determine the mechanical AWS Practical Reference Guide 3 STD*AWS PRGWM-ENGL 3999 W 07842b5 0539434 T2T Figure 4. Early a
36、cetylene welding equipment. properties so necessary for aiding the selection of alloys to perform a particular task. And lab instru- ments have been developed so the metals internal structure can be studied to increase our under- standing of their behavior. The metallurgical micro- scope and the sca
37、nning electron microscope (SEM) have contributed significantly to our basic under- standing of the metal structures. Metals are composed of many crystals or grains, with grain boundaries between the adjoining indi- vidual grains. These grains are usually quite small and require polishing and etching
38、 of the metal sur- face to be seen on a microscope at high magnifica- tions. Within each grain, an ordered structure exists and is comprised of millions of unit cells, each fol- lowing a natural physical order. At the junction of adjoining grains, there is a mismatch of each grains ordered structure
39、 since each grain is uniquely dif- ferent, and this boundary of mismatch delineates each individual grain. Figure 5 represents a 200X- 400X magnification of a metal structure showing portions of three grains and their adjoining bound- aries. Note the specific order within each grain that is unique t
40、o that grain alone. The ordered structure of unit cells within a metal grain can be represented by Figure 6, which repre- sents a three-dimensional metal structure using a ”unit cell” approach. This natural order is com- prised of metal atoms forming the unit cells. A unit cell is defined as ”The mi
41、nimum number of atoms that filly describe each of the unique structure orientations.” The sketched representation is only that; remember that the atoms are actually in motion with electrons encircling a core nucleus, and each element has a Figure 5. Representation of a metal structure showing portio
42、ns of three individual grains having an ordered structure within each, and the grain boundaries separating the grains, showing the orientation mismatch. unique core makeup with varying numbers of elec- trons encircling the nucleus. Metals usually fall into one of three types of unit cells or crystal
43、 structures: body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close- STD-AWS PRGWM-ENGL 1999 0784265 0539435 9bb A Figure 6. Isometric representation of the ordering of an array of unit cells and one unit cell from the array. packed (hcp). These three types are shown in Figure 7.
44、Each gets its name from the orientation of the atoms in the unit celi; ”cubic” refers to the atoms at the corners forming a cube, with each of its three axes identical in length. The terms ”body” or “face” refer to the position of the additional atoms not at the corners. For the bcc structure, an ad
45、ditional atom is at the exact center of the cube. For the fcc, additional atoms are positioned on each of the 6 faces, with each face atom being shared with ad- joining unit cells. The hcp structure is a bit more complicated, with 6 atoms forming each of the two end faces in a hexagonal shape, while
46、 the ”close- packed” term refers to the additional three atoms between the two hexagonal end faces. Metals and alloys can often exist with more than one crystal structure pattern, depending on the metals temperature and its rate of cooling to room temperature. There are various phase names at- tache
47、d to these different structures. Steel alloys can have unit cell orientations of bcc (ferrite phase), fcc (austenite phase), and an additional crystal struc- ture known as ”body-centered tetragonal” (bct) which is the structure of the martensite phase. The bct unit cell is very similar to bcc but wi
48、th one axis longer than the other two. The bct unit cell is shown in Figure 8, and it forms on rapid quenching for many carbon and low-alloy steels. A review of the iron-iron carbide diagram shown in Figure 9 shows the temperatures and carbon contents required for each of the various structures to b
49、e in stable forms. As seen in Figure 9, heating a 0.77% carbon steel above about 1333F causes it to transform quickly from bcc ferrite to fcc austenite. The reverse trans- formation occurs on slow cooling back to room temperature. Other carbon-percent alloys transform also, but over a temperature range rather than at a discrete temperature. The physical metallurgist takes advantage of these phase and structure trans- formations to develop quenching and tempering procedures to increase the mechanical strengths of alloys. Several metals have the fcc structur
