Tài liệu WELDING AND COATING METALLURGY ppt

Figure 5 Solubility of carbon in α bcc iron as a function of temperature In contrast the fcc form of iron dissolves up to 2% carbon, well in excess of the usual carbon content of steels.

4.3 EFFECT OF ALLOYING ELEMENTS 24

WELDING AND COATING METALLURGY2 12 October 1999 2 of 69

13.1 BASE METAL CONSIDERATIONS 49

WELDING AND COATING METALLURGY

1 INTRODUCTION

Steels form the largest group of commercially important alloys for several reasons:

♦ The great abundance of iron in the earth’s crust

♦ The relative ease of extraction and low cost

♦ The wide range of properties that can be achieved as a result of solid state transformation such as alloying and heat treatment

1.1 ROLE OF CARBON IN STEEL

Steels are alloys of iron with generally less than 1% carbon plus a wide range of other elements Some of these elements are added deliberately to impart special properties and others are impurities not completely removed (sometimes deliberately) during the steel making process Elements may be present in solid solution or combined as intermetallic compounds with iron, carbon or other elements Some elements, namely carbon, nitrogen, boron and hydrogen, form interstitial solutions with iron whereas others such as manganese and silicon form substitutional solutions Beyond the limit of solubility these elements may also form intermetallic compounds with iron or other elements Carbon has a major role in a steels mechanical properties and its intended use as illustrated in Figure 1

As the carbon concentration is increases carbon steel, in general, becomes stronger, harder but less ductile This is an important factor when a steel is required to be welded by joining or surfacing

WELDING AND COATING METALLURGY2 12 October 1999 4 of 69

Figure 1 Role of Carbon in Steel

Welding is one of the most important and versatile means of fabrication and joining available to industry Plain carbon steels, high strength low alloy (HSLA) steels, quench and tempered (Q&T) steels, stainless steels, cast irons, as well as a great many non-ferrous alloys such as aluminium, nickel and copper are welded extensively Welding is of great economic importance, because it is one of the most important tools available to engineers in his efforts to reduce production, fabrication and maintenance costs

A sound knowledge of what is meant by the word “weld” is essential to an understanding of both welding

and weldability A weld can be defined as a union between pieces of metal at faces rendered plastic or liquid by heat, or pressure, or both, with or without the use of filler metal Welds in which melting

occurs are the most common The great majority of steels welded today consist of low to medium carbon

steel (less than 0.4%C).Practical experience over many years has proved that not all steels are welded with ease For example, low carbon steels of less than 0.15%C can be easily welded by nearly all welding processes with generally high quality results The welding of higher carbon steels or relatively thick sections may or may not require extra precaution The degree of precaution necessary to obtain good quality welds in carbon and alloy steels varies considerably The welding procedure has to take into consideration various factors so that the welding operation has minimal affect on the mechanical properties and microstructure of the base metal

The application of heat, generally considered essential in a welding operation, produces a variety of structural, thermal and mechanical effects on the base metal being welded and on the filler metal being added in making the weld Effects include:

♦ Expansion and contraction (thermal stresses etc.)

♦ Metallurgical changes (grain growth etc.)

♦ Compositional changes (diffusion effects etc.)

In the completed weld these effects may change the intended base metal characteristics such as strength, ductility, notch toughness and corrosion resistance Additionally, the completed weld may include defects

such as cracks, porosity, and inclusions in the base metal, heat affected zone (HAZ) and weld metal itself

These effects of welding on any given steel are minimized or eliminated through changes in the detailed welding techniques involved in producing the weld

It is important to realize that the suitability of a repair weld on a component or structure for a specific service condition depends upon several factors:

♦ Original design of the structure, including welded joints

♦ The properties and characteristics of the base metal near to and away from the intended welds

♦ The properties and characteristics of the weld material

♦ Post Weld Heat Treatment (PWHT) may not be possible

As discussed, a steels weldability will be dependent upon many factors but the amount of carbon will be a principal factor A steels weldability can be categorized by its carbon content as shown in Table 1

WELDING AND COATING METALLURGY2 12 October 1999 6 of 69

Table 1 Common Names and their Typical Uses for Carbon Steel

HARDNESS

plate, bar

Good

(preheat & postheat normally required;

recommended)

(preheat and post heat; low H2

Figure 2 Transformation of crystal structure for iron showing contraction occurring at 910°C

Figure 3 BCC Crystal Structure

Figure 4 FCC Crystal Structure

Above 1390°C and up to the melting point at 1534°C the structure reverts back to body-centred cubic form These are known as allotropic forms of iron The face-centred cubic form is a close-packed structure being more dense than the body-centred cubic form Consequently iron will

actually contract as it is heated above 910°C when the structure transformation takes place

2.1 SOLUBILITY OF CARBON

The solubility of carbon in the bcc form of iron is very small, the maximum solubility being only about 0.02 wt.% at 723°C Figure 5 shows there is negligible solubility of carbon in iron at ambient temperature (less than 0.0001 wt.%) Since steels nearly always have more carbon than this, the excess carbon is not in solution but present as the intermetallic compound iron-carbide

Fe3C known as cementite

`

Figure 5 Solubility of carbon in α (bcc) iron as a function of temperature

In contrast the fcc form of iron dissolves up to 2% carbon, well in excess of the usual carbon content of steels A steel can therefore be heated to a temperature at which the structure changes from bcc to fcc and all the carbon goes into solution The way in which carbon is obliged to redistribute itself upon cooling back below the transformation temperature is the origin of the wide range of properties achievable in steels

3 IRON - IRON CARBIDE PHASE DIAGRAM

Fundamental to a study of steel metallurgy is an understanding of the iron – iron carbide phase diagram The diagram commonly studied is actually the metastable iron – iron carbide system The true stable form of carbon is graphite, but except for cast irons this only occurs after prolonged heating Since the carbon in steels is normally present as iron carbide, it is this system that is considered Figure 7 shows the iron – iron carbide system up to 6 wt.% carbon We will now consider several important features of this diagram

Figure 6 The iron-iron carbide equilibrium phase diagram

A eutectic is formed at 4.3% carbon At 1147°C liquid of this composition will transform to two solid phases (austenite + cementite) on cooling This region is important when discussing cast irons but is not relevant to steels

3.1 AUSTENITE (γ)

This region in which iron is fcc, identified in Figures 7 and 8, dissolves up to 2% carbon This phase is termed austenite or gamma phase With no carbon present it begins at 910°C on heating but with 0.8% carbon it starts at 723°C When a steel is heated into the austenite region all carbon and most other compounds dissolve to form a single phase (i.e normalizing)

Figure 7 The austenite region of the iron-iron carbide diagram showing

maximum solubility of up to 2%C

3.2 FERRITE (α)

The region shown in Figure 9 where carbon is dissolved in bcc iron is very narrow, extending to only 0.02% carbon at 723°C This phase is termed ferrite or alpha phase Although the carbon content of ferrite is very low other elements may dissolve appreciably in it so ferrite cannot be considered as “pure iron”

Figure 8 The ferrite region of the iron-iron carbide diagram

3.3 PERITECTIC

The region at the top left portion of the phase diagram enlarged in Figure 10 is where the iron reverts back to the bcc structure known as delta ferrite Here again the solubility for carbon is low, only 0.1 wt.% at 1493°C The part of the diagram at 0.16% carbon having the appearance of

an inverted eutectoid is called a peritectic At this point a two phase mixture of liquid and solid (austenite) transforms on cooling to a single phase solid of austenite This portion of the phase diagram will not be discussed in detail, but it should be recognized since it has been invoked to explain various hot cracking phenomena in welding

Figure 9 Peretectic region of the iron-iron carbide diagram

3.4 PEARLITE

At 0.8% carbon and 723°C a eutectoid is formed as illustrated in Figure 11 This is similar to the eutectic transformation but involves a solid phase transforming into two different phases on cooling (ferrite and cementite) This eutectoid mixture is called pearlite Figure 12 shows how the two phase constituents that make up pearlite are formed Note that pearlite is only one of many phases that can be produced from ferrite and cementite (depending on cooling rate) Cementite (iron carbide) itself is very hard - about 1150 Hv – but when mixed with the soft ferrite layers to form pearlite, the average hardness of pearlite is considerably less

Figure 10 The eutectoid point on the iron-iron carbide diagram

Figure 11 Schematic View of how pearlite is formed in an approx 0.4%C steel

This region of the phase diagram (where carbon concentration is less than 0.8%) is of the most interest to a study of steels and their weldability which will be discussed in more detail later

A steel with 0.8 wt.% carbon, it will be recalled, transforms on cooling through 723°C to the two phase eutectoid constituent pearlite In pearlite the two phases ferrite and cementite are mixed closely together in fine layers As the ferrite contains very little carbon while the cementite has 6.7%, carbon atoms must diffuse to the growing cementite plates as shown in Figure 13

Figure 12 Schematic View of different pearlite growth rates

The distance they can diffuse, and hence the spacing of the plates, depends on how fast the pearlite

is growing A fast growth rate means less time for diffusion and a finer pearlite results Figure 14 shows a typical pearlite microstructure

Figure 13 Typical Lamellar Appearance of Pearlite Mag:X1500

3.5 PRO-EUTECTOID FERRITE

If the steel has less than 0.8 wt.% carbon (termed hypo-eutectoid steel) ferrite will be formed first from the austenite The example in Figure 15 shows a steel of 0.4 wt.% carbon This ferrite is called pro-eutectoid ferrite because it transforms first on cooling as illustrated in Figure 15 As transformation continues and the temperature drops, the remaining austenite becomes richer in carbon At 723°C the steel comprises ferrite and the remaining austenite (which contains 0.8wt.% carbon) With further cooling, the austenite then transforms to pearlite producing a final structure

in the steel of pro-eutectoid ferrite and pearlite

The amounts of pro-eutectoid ferrite and pearlite can be estimated by application of the lever rule (see references for more detailed information) For a 0.4 wt.% carbon steel about 50% will be ferrite and 50% pearlite Similarly a steel of more than 0.8 wt.% carbon (from 0.8 wt.% up to 1.8 wt.% carbon is termed hyper-eutectoid steel) first transforms to cementite (i.e pro-eutectoid carbide) with the remaining austenite forming pearlite as shown in Figure 16

Figure 15 Phase Transformation on Cooling a 1.2%C Steel

3.6 PHASE TRANSFORMATIONS IN LOW ALLOY STEELS

Figure 17 shows the appearance of a polished and etched section of an approximately 0.6wt.% carbon steel You can see that the pro-eutectoid ferrite has formed initially at the austenite grain boundaries, nucleation taking place at several points around each austenite grain Since each region of ferrite becomes an individual grain, its grain size will be very much smaller than that of the parent austenite Ferrite continues to form and grow until the final transformation of remaining austenite to pearlite The ferrite does not always appear as neat, equiaxed grains as shown in Figure 17, but can occur as long spikes from the grain boundaries or even nucleate within the austenite grain This can occur quite markedly from the welding process due to the cooling rates imposed by the heat input (i.e travel speed)

Figure 16 Prior Austenite Boundaries Showing Pro-Eutectoid Ferrite

On reheating the steel the process reverses and the pearlite and ferrite grains transform back into single phase austenite to form completely new grains The temperature required to get complete transformation depends on the carbon level as seen from the phase diagram (see Figures 7 and 15)and ranges from 910°C for zero carbon to 723°C for 0.8 wt.% carbon

3.7 GRAIN GROWTH

Heating to higher temperatures than those necessary to get complete transformation causes the austenite grains to grow The final size of the austenite grains depends not only on the temperature reached but also on the type of steel Some steels containing small precipitates such

as aluminium and vanadium nitride retain small grain size up to high temperatures These are known as fine grained steels Steels can be deliberately made as coarse grain or fine grain Fine grained steels are tougher and are more commonly specified for most structural applications

The effect of austenizing temperature on grain size is shown in Figure 18 It shows that although grain growth is restricted in a fine grain steel, at a sufficiently high temperature the precipitates dissolve and the steel behaves as a coarse grain steel Thus at sufficiently high temperature, grain growth can occur with subsequent loss of toughness This is an important consideration in the HAZ associated with welding

Figure 17 Schematic Effect of Temperature on Grain Growth for Coarse and Fine Grained Steels

3.8 NON-EQUILIBRIUM COOLING

The phases and microstructures predicted by the iron – iron carbide diagram occur in steels cooled very slowly In addition the diagram assumes that carbon is the only alloying element present in the steel With the addition of other common alloying elements such as manganese, silicon, nickel, titanium, molybdenum, chromium etc., the phase diagram can still be used except that it will be distorted and the lines may move to slightly different locations

Figure 18 Effect of Various Element Additions on the Recrystallization Temperature

For example the presence of alloy elements changes the recrystallization (eutectoid) temperature

as shown in Figure 19 In structural steels the concentration of alloys is generally quite small (austenitic manganese steels are an exception containing over 12 wt.% manganese) and the basic iron – iron carbide phase diagram is not distorted very much from equilibrium conditions

3.9 MARTENSITE - EFFECT OF RAPID COOLING

The rate of cooling has a major effect on the types of microstructures formed and unless the steel cools slowly the iron – iron carbide phase diagram cannot be used The reason is that the transformation of austenite to pearlite requires the diffusion of carbon to the sites of growing carbon, a process which takes time We saw how a faster cooling rate produced finer pearlite With even faster cooling rates less time is available for diffusion and pearlite cannot form Alternative microstructures form with their exact morphology depending on just how quickly the steel cools In a water quench, for example, the cooling rate is so rapid there is no time for any diffusion, and the carbon remains trapped in the same place as it was in the austenite A rapid quench cannot suppress the crystal structure change from fcc to bcc but the presence of trapped carbon in the bcc phase distorts it to a tetragonal shape, as indicated in Figure 20, rather than a true cubic structure This is called martensite

Figure 19 Schematic Transformation of Austenite (BCC) To Martensite (Tetragonal) With Increasing %C

The amount of carbon influences the amount of distortion in the crystal structure as shown in Figure 20 This in turn affects the hardness of the martensite as shown in Figure 22

Under the microscope as shown in Figure 21 martensite has the appearance of a mass of needles

Figure 20 Martensite Microstructure

Martensite can be very hard and brittle when it contains appreciable amounts of carbon The hardness

depends almost exclusively on the carbon

content with other elements having little effect as illustrated in

Figure 22

Figure 21 Effect of Carbon and Alloying on the Hardness of Martensite

The formation of martensite can occur in the HAZ adjacent to a weld deposit due to the fast cooling rates imposed by the welding process This is discussed in more detail in Section !!

Bainite is still a two phase mixture of ferrite and iron carbide but unlike the cementite plates in pearlite the carbide in bainite is spherical Bainite formed above 300°C contains relatively coarse particles of the Fe3C form of iron carbide (cementite) and is termed upper bainite When formed

below 300°C bainite has a much finer structure with the carbides tending to form striations across

the ferrite laths This is termed lower bainite The carbides in lower bainite are Fe2.4C known as epsilon (ε) carbide Some steels in the bainitic condition may possess ductility and toughness superior to that shown by the same steel in the Q&T condition

4 TRANSFORMATION DIAGRAMS

Since the iron – iron carbide phase diagram is only valid for very slow cooling rates, alternative diagrams for determining the constituents present in a more rapidly cooled steel have been developed There are two types:

♦ Time Temperature Transformation (TTT) curves where the steel sample is held at a constant temperature until transformation is complete

♦ Continuous Cooling Transformation (CCT) curves where the steel sample is cooled from the austenitic region at different cooling rates

Although these diagrams are principally designed for the foundry metallurgist and heat treater etc., they are an excellent tool for use by welding engineers where fast cooling rates need to be evaluated near to the welded area

4.1 TIME TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS

Consider heating a sample of steel until it is fully austenitic then quenched to some temperature below the equilibrium transformation temperature as shown in Figure 24

Figure 23 Schematic Representation of TTT

If we hold the steel at this temperature we find there is a delay before transformation begins and a further elapse of time while transformation takes place The delay depends on the temperature at

which the steel is held and we can plot this information on a diagram of temperature against time for a given steel composition

Figure 24 Schematic TTT Curve for Carbon Steel

An example of such a time-temperature-transformation (TTT) diagram for a carbon steel is shown

in Figure 25 Note that at high temperatures (Figure 26) the steel transforms to pro-eutectoid ferrite followed by pearlite

Figure 25 TTT Curve Illustrating High Temperature Transformation of Pro-Eutectoid Ferrite

At lower temperatures less pronounced pro-eutectoid ferrite is formed and the pearlite is finer At about 550°C the pearlite forms in the shortest time and there is no pro-eutectoid ferrite (Figure

27)

Figure 26 TTT Curve Illustrating Pearlite Transformation

Cooling down to below this range (approximately 450°C) transformation to bainite occurs, taking

a longer time for lower temperatures (Figure 28)

Figure 27 TTT Curve Illustrating Transformation to Bainite

At a sufficiently fast cooling down to low temperature martensite can begin to form (Figure 29) Note that it forms almost instantaneously and does not grow as a function of time For each steel specification there is a fixed temperature Ms at which martensite starts to form and a fixed temperature Mf at which transformation is complete The percentage of martensite formed therefore depends only on the temperature to which the steel is rapidly cooled c and not on how long it is held there If the composition of the steel is known, the Ms temperature can be calculated

(see Section !!!) Note that for some compositions the Mf temperature can be below ambient temperature

Figure 28 TTT Curve Illustrating Martensite Formation

4.2 CONTINUOUS COOLING TRANSFORMATION (CCT) DIAGRAMS

Now consider the case of continuous cooling We may superimpose a cooling curve on the TTT diagram as illustrated in Figure 30 in order to get an idea of what microstructures form, but it is more accurate to use a diagram established under continuous cooling conditions The CCT diagram is slightly different from the TTT curve

Figure 29 Cooling Curves Superimposed onto TTT Curve for Typical Carbon Steel

4.2.1 CRITICAL COOLING RATES

You should note that in plain carbon steels bainite generally will not form during continuous cooling because of the shape of the TTT diagram The bainite region is tucked under the pearlite area so a cooling curve either hits the pearlite curve or misses it completely as shown in Figure 30

At cooling rates fast enough to miss the nose of the curve martensite is formed

This is an important concept since the cooling rate at which martensite can form in a HAZ strongly influences the risk of cracking during welding and gives an indication of a steels

“weldability” This will be discussed in more detail in Section 8

4.2.2 DETERMINING CCT DIAGRAMS

The exact shape of a CCT curve depends on the chemistry of the steel and on the heating and cooling cycles CCT diagrams are available for numerous carbon and alloy steels and if desired can even be established for specific weld metal

4.3 EFFECT OF ALLOYING ELEMENTS

Alloy elements have significant effects on the shape of the CCT and TTT diagrams which allow different microstructures to be produced in alloy steels Chromium and molybdenum, for example, shift the top (pearlite) part of the curve to the right i.e to longer times, thus exposing the bainite region Steels containing these elements such as 4135 can produce bainite on continuous

cooling

Figure 30 CCT Curve for 4135 Steel

The entire TTT curve may also shift to the right with additions of certain elements (e.g chromium, vanadium, molybdenum and others) to greater times allowing martensite to form at much slower cooling rates This increases the “hardenability” of the steel, but also increases the risk of cracking from welding if proper precautions are not taken

4.4 M S AND M F TEMPERATURES

The other notable effect of alloy element addition is to change the martensite start (Ms) and martensite finish (Mf) temperatures Increasing the carbon content, for example, depresses the Ms

to lower temperatures as shown in Figure 32

Figure 31 Schematic Diagram Showing the Influence of %C on Martensitic Start Temperature

Other elements affect martensite formation and the combined affect can be approximated by the following equation:

M s (°C) = 550 – 350 ×%C - 40×%Mn - 35×%V - 20×%Cr - 17×%Ni -10×%Cu - 10×%Mo - 5×%W + 15×%Co +

5 HARDENABILITY / WELDABILITY OF STEELS

Figure 32 Correlation of CCT and TTT Diagrams With Jominy Hardenability Test Data

for an 8630 Type Steel

The hardenability of steels can be determined by performing a Jominy end quench test The alloy

steel test specimen is a cylinder one inch diameter and four inches long, which is heated to the austenitic region (above 910°C) then placed in a fixture where it is quenched by water or brine impinging on one end The fastest cooling rate occurs at the bar surface in contact with the water jet with progressively slower cooling rates being experienced away from the end Thus the microstructure formed in the surface region could be martensitic with high hardness and the interior could be pearlitic with no hardening at all The depth to which a steel hardens is a measure

of its hardenability If we add alloying elements that allows deeper hardening, then that steel is said to have higher hardenability This is important, for example, when considering mechanical properties and weldability of such a steel Hardness tests are commonly used on Jominy samples

to determine that steels hardenability

Figure 33 illustrates the TTT diagram for a common chrome-molybdenum steel (4137) with a Jominy end quench test superimposed Thus the microstructure and hardness can be correlated on the one diagram

The cooling rate curves represent the same cooling rate conditions located along the Jominy quench test bar At the top of Figure 33, the measured hardness curve has been superimposed over

end-a schemend-atic of the end-quenched bend-ar Four representend-ative locend-ations (A, B, C, D) end-along the bend-ar hend-ave been related to the representative cooling curves(CCT) and isothermal transformation (TTT) curves Thus location A on the bar experienced a fast cooling rate resulting in austenite transforming to martensite producing the high hardness indicated Similar cooling rate effects need to be considered from a weldability viewpoint

The addition of alloying elements (for example Mo, Cr, Mn) to steel increases the hardenability by slowing down the rate of austenite transformation The data is plotted as shown in Figure 34 for a 0.45%C steel with different alloying additions

Figure 33 Typical End-Quench Curves for Several 0.45%C Low Alloy Steels

Several formulae have been developed which assign a contributing factor to each element addition and its effect on hardenability and conversely weldability The maximum hardness attainable (and therefore its weldability characteristics) in carbon and low-alloy steels, however, is still almost exclusively dependent upon the carbon content

5.1 CARBON EQUIVALENT (CE) & WELDABILITY

Depth of hardening is not a relevant concept in a welding situation, but we are interested in the hardness produced at a given cooling rate or the critical cooling rate to produce a given hardness

in the HAZ of a weld There are several models that have been developed to calculate hardenability from a welding process The simplest model is one in which the effects of individual

alloying elements are added together (a linear model) to produce a carbon equivalent (CE) which

in turn relates to a critical cooling rate to produce a given hardness Figure 35 shows a reasonable correlation between the CE plotted against critical cooling rate from 540°C to give a hardness of 350Hv in the HAZ

Figure 34 Linear Correlation of CE and Cooling Rate for a Fixed HAZ Hardness

Another linear model has been used to predict the hardness of the HAZ for different cooling rates

in low alloy steels and is illustrated in Figure 36

0 200 400 600 800 1000 1200 1400

Hv@50deg/sec Hv@100deg/sec Hv@200deg/sec Hv@500deg/sec

Figure 35 Correlation of HAZ Hardness and CE as a Function of Cooling Rate

CE’s are used widely in industry as measures of weldability Several different formulae have been developed and some are even incorporated into national codes and specifications In general terms, other factors being equal, as the carbon content increases, so does the difficulty in weldability In practice, this means generally using higher preheats until cracking and restraint problems are overcome

Using an engineering/analytical approach becomes very useful when confronted with unknown material compositions, and weld repairs can become challenging where reverse engineering must

be utilized to develop a repair procedure The engineering approach may involve evaluating composition, hardenability, service conditions, size, restraint conditions, and PWHT feasibility

One of the popular methods for determining weldability is to review the hardenability of the base material As discussed earlier the CE formula(s) have been developed as a convenient method of normalizing the chemical composition of a material into a single number to indicate its hardenability Review of the literature indicates no less than a dozen different formulas have been developed One of the most commonly used formulas for calculating the CE is the IIW formula (shown in Figure 36):

15

Cu Ni 5

V Mo Cr 6

AWS D1.1, Structural Welding Code – Steel The CE is not usually evaluated on these materials

Medium carbon, HSLA, and Q&T Steels, however, present different challenges where consideration of CE, restraint, hydrogen control, PWHT not practicable, weld filler chemistry mismatch, weld heat input etc can be critical to successful repair welding These factors can be summed up as a materials weldability, and it is these factors that will be considered in Section 8

5.2 TEMPERING – EFFECTS OF REHEATING

As discussed earlier martensite produced in a quenched steel is hard and brittle and in most cases

the steel is unusable in that form The toughness may be improved by a process of tempering This

involves reheating the steel to below the transformation temperature (723°C), holding for a period

of time, then cooling to ambient temperature as illustrated in Figure 37 During tempering the carbon trapped as an interstitial in the martensitic tetragonal structure is released Carbon atoms diffuse and precipitate as small carbides With enough time and at sufficiently high temperatures cementite (Fe3C) forms, not as plates as in pearlite, but as spherical particles This microstructure

is known as bainite(see section 3.9)

Figure 36 TTT Curve illustrating Q&T process to form Bainite

Improvement in toughness is accompanied by a loss of hardness which is a function of both temperature and time (however temperature is more effective – the higher the temperature the faster the tempering transformation as illustrated in Figure 37) The temperatures typically selected for post weld heat treating or stress relieving welded steel are generally high enough to cause rapid tempering of the HAZ

5.3 SECONDARY HARDENING

In some steels containing specific alloy elements tempering may actually cause an increase in hardness as the tempering temperature is raised as shown in Figure 38 This is known as

secondary hardening and is caused by strong carbide forming elements such as molybdenum,

chromium, and tungsten combining with carbon to form alloy carbide precipitates in certain temperature ranges This behavior of secondary hardening is put to good use in the tempering of tool steels such as high speed tool steels When considering a weld repair on such steels, the preheat and interpass temperatures is normally selected at a temperature below the secondary(or tempering) temperature, particularly if PWHT is not practical

Figure 37 Alloying Effect on Secondary Hardening

6 HYDROGEN CRACKING RELATED TO WELDABILITY

Hydrogen can embrittle a steel at both elevated and ambient temperatures The term hot cracking

is used to signify that cracking has occurred at elevated temperature while cold cracking is used to

generally signify cracking in low alloy steel at ambient temperature It was during World War 2 that it was realized that hydrogen dissolved in weld metal was one of the causes of cold cracking

in low alloy steel welded joints (i.e the catastrophic failure of the welded Liberty ships) These failures led to the development of low hydrogen electrodes which made possible successful welding of the alloy steels used today

Hydrogen pickup is derived from hydrogen containing chemical compounds that are dissociated in the arc column They can originate, for example, from contamination on the workpiece or from moisture in the welding flux It is the hydrogen sourced from electrode coatings or fluxes which is the most important Electrode coatings consist of minerals, organic matter, ferro-alloys, and iron powder bonded with, for example, bentonite (a clay) and sodium silicate The electrodes are baked after coating, and the higher the baking temperature the lower the final moisture content of the coating Some electrode coatings may pick up moisture if exposed at ambient conditions (basic coated electrodes) Where hydrogen cracking is a risk, special flux coatings are used to maintain low hydrogen content In practice, welding specifications stipulate the allowable moisture content It is, however, important to note that the method or welding procedure adopted as well as the type of electrode flux used can affect the hydrogen content in a weld or HAZ

With hot cracking, embrittlement occurs in carbon and low alloy steels by a chemical reaction occurring between hydrogen and carbides which causes irreversible damage – either decarburization or cracking or both Of much greater importance in welding is hydrogen entrapped in the weld or HAZ causing embrittlement Hydrogen cracking can subsequently occur

at some later time (sometimes days) once a weld repair is complete, generally at service temperatures between – 100°C and 200°C This embrittlement is due to physical interactions between hydrogen and the crystal lattice structure of the steel and is reversible by removal of hydrogen by stress relieving allowing the ductility of the steel to revert back to normal Hydrogen cracking can occur in either the weld metal, HAZ, or base metal and be either transverse or longitudinal to the weld axis The level of preheat or other precautions necessary to avoid cracking will depend on which region is the more sensitive In carbon - manganese medium strength steels the HAZ is usually the more critical region and weld metal rarely causes a problem

Cracking due to dissolved hydrogen is now thought to occur by decohesion Where there is a defect, discontinuity or pre - existing crack and a tensile stress applied, hydrogen is considered to diffuse preferentially to the region of greatest strain i.e near to the stress concentration such as near a crack tip The presence of a relatively large concentration of hydrogen reduces the cohesive energy of the crystal lattice structure to the extent that fracture occurs at or near the stress concentrator This view is consistent with observations that cracking can occur slowly (the crack velocity being dependent on the diffusion rate of hydrogen) and is quite often discontinuous

In welding, the region most susceptible to hydrogen cracking is that which is hardened to the highest degree (areas where the welding residual stresses is greatest) although regions of coarse grain growth can be a contributing factor The most crack-sensitive microstructure is high carbon martensite

Hot or cold cracking in the weld metal or HAZ depends on the same fundamental factors as in the base metal, i.e hydrogen content, microstructure and residual stress In practice the controlling variables are usually strength, hydrogen content, restraint, stress concentrations, and heat input

Figure 38 Minimizing Heat Input by Multi-Pass Welding

In single pass welds and root runs of multiple pass welds the root pass may provide a stress

concentration which can lead to longitudinal cracks in the weld metal High dilution of the root run (high heat input) can often result in a harder weld bead more likely to crack (this is commonly seen in such applications as pipeline welding) Figure 40 illustrates the physical appearance of hydrogen cracking in welds

Figure 39 Schematic View of Typical Weld Cracks

Figure 40 Cracking Caused By Lack Of Fusion in Weld

In Figure 41 the crack has initiated at the root of the weld where a lack of fusion can be seen The crack has then traveled through the HAZ mainly in the coarse grained region In heavy multiple – pass welds cracking will generally be transverse to the weld direction, sometimes running through the weld itself since the maximum cooling rate is along the weld axis Many HSLA steels in critical repair situations where PWHT is impracticable are welded using a filler metal of good toughness and ductility and in such cases the HAZ may be more crack sensitive

The risk of hydrogen-induced cold cracking in the weld can be minimized by:

♦ Reducing hydrogen pick-up (low hydrogen flux chemistries)

♦ Maintaining a low carbon content

♦ Avoiding excessive restraint

♦ Control of welding procedures (preheat; heat input; PWHT etc.)

In carbon or carbon-manganese steels (i.e., those with steep hardening curves as shown in Figure 35!!) welding conditions can be selected to avoid the cooling rates at which martensite is produced This could include preheat; high heat input welding; slow cooling etc

In low alloy steels or those where a hard HAZ cannot be avoided, other steps must be taken to prevent cracks These often involve applying preheat and interpass temperatures to allow the diffusion of hydrogen out of the weld metal Figure 43 shows that quite moderate temperatures are highly effective in removing hydrogen

Figure 41 Effect of Moderate Postheat on Hydrogen Content in a Cooled Weld

The freedom of selecting a suitable welding solution is sometimes limited The solution must be practicable and economic Further constraints may be applied by the job such as base metal condition, size, location, PWHT not practicable, equipment availability etc In such cases, the welding engineer may need to consider the steels CE and Ms temperature by referring to its TTT and CCT curves in providing a weld procedure

6.1 LAMELLAR TEARING

Lamellar tearing is a form of cracking that occurs in the base metal of a weldment due to the combination of high localized stress and low ductility of the base metal It is associated with regions under severe restraint, for example, tee and corner joints; heavy sections etc

Figure 42 Example of Lamellar Tearing from Welding