Thermit Welding

Thermit welding is basically a fusion process, the required heat being evolved from a mixture of powdered aluminum and iron oxide.

From: Ship Construction (Seventh Edition), 2012

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Physics of welding

Ramesh Singh, in Applied Welding Engineering (Third Edition), 2020

Thermit welding

TW is a process that uses heat from exothermic reaction to produce coalescence between metals. The name is derived from “thermite,” which is the generic name given to a reaction between metal oxides and reducing agents. The thermite mixture consists of metal oxides with low heats of formation and metallic-reducing agents that, when oxidized, have high heats of formation. The excess heats of formation of the reaction products provide the energy source to form the weld.

Let us consider an example; if fine aluminum particles and metal oxides are blended and ignited by an external heat source, the aluminothermic reaction will precede according to the following equation:

Metal Oxide+aluminum→aluminum oxide+metal+heat

The most common thermit welds reactions are listed in the following table:

[12]¾Fe3O4+2Al→9/4Fe+Al2O3ΔH=838kJ
[13]3FeO+2Al→3Fe+Al2O3ΔH=880kJ
[14]Fe2O3+2Al→2Fe+Al2O3ΔH=860kJ
[15]3CuO+2Al→3Cu+Al2O3ΔH=1210kJ
[16]3Cu2O+2Al→6Cu+Al2O3ΔH=1060kJ

The theoretical estimated maximum temperature from the reactions listed earlier is 3200°C (5800°F). In practice, however, the heat ranges between 2200°C (4000°F) and 2400°C (4350°F). The ignition temperature of the thermit granules used for welding is about 1200°C (2200°F); therefore, it is safe from fire hazards if stored away from open heat sources.

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Physics of Welding

Ramesh Singh, in Applied Welding Engineering, 2012

Thermit Welding

Thermit welding (TW) is a process that uses heat from an exothermic reaction to produce coalescence between metals. The name is derived from ‘thermite’ the generic name given to reactions between metal oxides and reducing agents. The thermite mixture consists of metal oxides with low heats of formation and metallic reducing agents which, when oxidized, have high heats of formation. The excess heats of formation of the reaction products provide the energy source to form the weld.

Let us consider an example; if fine aluminum particles and metal oxides are blended and ignited by an external heat source, the aluminothermic reaction will precede according to the following equation:

Metal oxide+aluminum→aluminum oxide+metal+heat

The most common thermit welds reactions are listed in the following table:

(12)34Fe3O4+2Al→94Fe+Al2O3 (ΔH=838kJ)
(13)3FeO+2Al→3 Fe+Al2O3 (ΔH=880kJ)
(14)Fe2O3+2Al→2 Fe+Al2O3 (ΔH=860kJ)

(15)3CuO+2Al→3Cu+Al2O3(ΔH=1 210kJ)
(16)3Cu2O+2Al→6Cu+Al2O3 (ΔH=1 060kJ)

The theoretical estimated maximum temperature of the reaction listed above is 3,200°C (5,800°F). In practice however, the heat ranges between 2,200°C (4,000°F) and 2,400°C (4,350°F). The ignition temperature of the thermit granules used for welding is about 1,200°C (2,200°F), therefore it is safe from fire hazards if stored away from open heat sources.

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Commissioning of Cathodic Protection Systems

Alireza Bahadori Ph.D., in Cathodic Corrosion Protection Systems, 2014

8.6 Thermit Welding of CP Leads

This Clause 13 covers the connection of CP wire leads to new or in-service carbon steel pipelines under pressure by thermit welding. Connections to pipe less than 3 mm thick should be made using approved clamps or silver soldering.

For the purposes of this standard, thermit welds, thermowelds, and cadwelds are synonymous.

Thermit weld process should be applied only by skilled experienced field personnel.

Thermit welds should not be made on internally plastic-coated pipelines. Internal coatings affected include epoxies, phenolics, nylon, polyethylene liners, etc.

The thermit weld charge should be limited to Thermoweld Cartridge No. 15 F33 (15 g), or equivalent. The maximum size of the electrical conductor should be 25 mm2 (No. 6 AWG). Where the attachment of a larger conductor is required, a multistrand wire should be used and the strands should be arranged into groups no larger than 25 mm2 (No. 6 AWG) and each group attached to the pipe separately.

The minimum distance of a thermit weld from a circumferential weld should be 200 mm.

The minimum distance of a thermit weld from a longitudinal weld should be 40 mm.

In attaching one wire to a pipeline, only one charge should be used. If the first thermit weld does not take, a second thermit weld should not be attempted on the same spot.

If a thermit weld is disapproved on the first charge, it should either be removed, the surface cleaned to bright metal, and the process is repeated or a new location on the pipe is selected.

The permissible operating pressure in the pipeline when thermit welding is shown in Table 8.1. If the pipe on which the thermit weld will be attached is not in the table, the permissible operating pressure can be calculated using the following formula:

Table 8.1. Permissible Hot Work Pressure While Thermit Welding

NPA Grade Specified Minimum Yield OD WT Permissible Hot Work Pressure
CSA API MPa PSI mm in mm in kPa Psig
2 207 A 207 30,000 60.3 2.375 3.91 0.154 11,529 1664
241 B 241 35,000 60.3 2.375 3.18 0.125 9170 1330
3 241 A 241 35,000 88.9 3.5 3.18 0.125 6205 900
290 X42 290 42,000 88.9 3.5 4.78 0.188 15,059 2169
4 241 B 241 35,000 114.3 4.5 3.18 0.125 4826 700
290 X42 290 42,000 114.3 4.5 4.78 0.188 11,713 1687
6 207 A 207 30,000 168.3 6.625 4.78 0.188 5678 818
241 B 241 35,000 168.3 6.625 4.78 0.188 6610 955
290 X42 290 42,000 168.3 6.625 3.18 0.125 3930 570
8 207 A 207 30,000 219.1 8.625 5.56 0.219 5427 783
241 B 241 35,000 219.1 8.625 8.18 0.322 10,486 1516
290 X42 290 42,000 219.1 8.625 4.17 0.164 4977 722
10 241 B 241 35,000 273.1 10.750 6.35 0.25 6077 879
317 X46 317 46,000 273.1 10.750 5.56 0.219 6668 964
317 X46 317 46,000 273.1 10.750 4.78 0.188 5308 770
12 290 X42 290 42,000 323.9 12.750 6.35 0.25 6166 889
317 X46 317 46,000 323.9 12.750 6.35 0.25 6740 974
359 X52 359 52,000 323.9 12.750 4.78 0.188 5060 734
14 290 X42 290 42,000 355.6 14.00 6.35 0.25 5616 810
317 X46 317 46,000 355.6 14.00 5.59 0.219 5122 743
16 317 X46 317 46,000 406.4 16.00 6.35 0.25 5372 776
359 X52 359 52,000 406.4 16.00 5.59 0.219 5067 735

CSA-Canadian Standards Association; OD-Outside diameter; WT- Wall Thickness; PSI-Pound per square inch; Psig-Pound per square inch gauge

(8.1)Pp=2S(t−1.59)×0.72×103D

Pp = Permissible “hot work” pressure (kPa).

S = Specified minimum yield strength of the pipe (MPa).

D = Nominal outside diameter of the pipe (mm).

t = Nominal wall thickness of the pipe (mm).

Note: S, D, and t values are to be obtained from the operating permits for the pipeline.

Thermit welds should not be attached to pipe wall thicknesses less than 3.18 mm while the pipeline is pressurized.

The connection of leads to high-pressure gas lines with wall thickness less than 4.78 mm, but more than 3.18 mm, by thermit welding should be done with the gas flowing.

The use of thermit welds should be avoided in high-stress areas such as elbows, tees, etc.

If more than one weld is required such as two adjacent wires or large conductor split in two to get the required size for a 15 g charge, the spacing between point of connection should not be less than 100 mm (4 in).

A suitable copper sleeve for smaller size wire should be used for a 15 g one-shot mold and the wire bended around the end of the sleeve.

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Welding and Bonding Technologies

F. Yusof, M.F. Jamaluddin, in Comprehensive Materials Processing, 2014

6.07.3.6 Railways

The welding of railway tracks involves a number of technological differences as compared to conventional welding processes (71). Flash butt welding (FBW) and aluminothermic welding (ATW) are commonly used in the welding of rail tracks. Rail steels contain relatively large amounts of alloying elements, particularly carbon (72). Due to the higher level of carbon, there is a tendency to form weld cracking and brittle zones in the weld area. Fractures originate more frequently from ATW than from FBW, and they generally arise from porosity, LOF, or hot tears within the WM.

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Welding and cutting processes used in shipbuilding

D.J. Eyres M.Sc., F.R.I.N.A., G.J. Bruce M.B.A, F.R.I.N.A., MSNAME., in Ship Construction (Seventh Edition), 2012

Thermit welding

This is a very useful method of welding that may be used to weld together large steel sections, for example parts of a stern frame. It is in fact often used to repair castings or forgings of this nature. Thermit welding is basically a fusion process, the required heat being evolved from a mixture of powdered aluminum and iron oxide. The ends of the part to be welded are initially built into a sand or graphite mold, whilst the mixture is poured into a refractory lined crucible. Ignition of this mixture is obtained with the aid of a highly inflammable powder consisting mostly of barium peroxide. During the subsequent reaction within the crucible the oxygen leaves the iron oxide and combines with the aluminum producing aluminum oxide, or slag, and superheated thermit steel. This steel is run into the mold, where it preheats and eventually fuses and mixes with the ends of the parts to be joined. On cooling a continuous joint is formed and the mold is removed.

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MECHANICAL ELECTRICAL CONNECTIONS

Robert W. MesslerJr., in Integral Mechanical Attachment, 2006

12.2.1 Welding

For connections intended to be permanent, welding is the joining process of preference. The problems are: (1) rarely is anything truly needed to be permanent (either because it needs to be repaired or because it eventually becomes obsolete) and (2) usually joints need to be produced in environments unsuited to the types of welding processes preferred for making electrical joints. Often, either access to the point requiring connection is limited or obtaining the level of process control needed to ensure a high-quality joint is produced is difficult to impossible to achieve.

The types of connections that are most frequently welded are copper bars, large-diameter (over 1/4 inch or 6 mm) solid wires, and bundled-wire cables to one another, as well as copper conductors to steel rails for grounding. The process employed has been and continues to be thermit welding (TW). Also known as “aluminothermic welding,” TW involves a highly exothermic reaction between finely-divided aluminum metal and fine, powdered copper oxide. Upon reaction, the Al reacts with the CuO/Cu2O to produce molten Cu metal along with solid Al2O3 as a dross. The process can be performed outdoors, on-site, but the need for full access to the intended joint limits field utility. Variations on the basic class of exothermic chemical reactions involved allow the welding of copper to copper, copper to aluminum, or copper to steel, and of aluminum to aluminum. Examples are shown in Figure 12.1.

FIGURE 12.1. A photograph of Cu-Cu welds (left) and Al-Al welds (right) made by the thermit welding process for electrical connection of cables to buss bars for high current applications.

(Photograph by Sam Chiappone for the author; used with permission.)

For smaller-diameter conductive elements, welds are made by percussion welding (PEW) and ultrasonic welding (USW). PEW is a resistance welding process also known, rather more descriptively, as “capacitor-discharge welding.” In this process, electrical energy stored in a capacitor is suddenly discharged to flow through two pieces, usually a wire to a plate. Rapid and highly-localized I2R heating at the high-resistance point of initial contact leads to localized melting and the formation of a weld. USW is a form of friction welding in which the high-frequency (over 25,000 cycles per second), small-amplitude movement of one piece in contact with another under pressure leads to sufficient heat to cause localized melting and the formation of a weld.

Beyond these processes, welding, as such, is not performed to produce electrical connections.

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Processing Metals 2

Michael F. Ashby, David R.H. Jones, in Engineering Materials 2 (Fourth Edition), 2013

Worked Example

New railroad rails are usually transported to the site in the form of long lengths (≈400 m), previously assembled by welding shorter lengths of rail end to end. These joints are done by flash butt welding at the workshops http://www.youtube.com/watch?v=8HOtXj0qlG8. This involves heating the two rail ends with a large low voltage DC current (which initially arcs across a narrow gap between the ends), then pressing the ends together (with the current still flowing) to make a hot forged joint across the full cross section of the rail. Once at the site, the ends of the long rail lengths are joined by aluminothermic welding (also called thermit welding). The geometry of this process is shown in the following diagram and described in the link http://www.thermit-welding.com/thermit_welding_process.php.

Schematic of thermit welding mold, pouring, and feeding geometry. The plug is inserted after preheating the rail ends. The Al2O3 slag floats to the top and overflows into slag trays. See http://www.youtube.com/watch?v=19jqMckL8bA.

The ends of the rails to be joined are spaced apart by about an inch and enclosed by a factory-made split ceramic mold. The rail ends are then preheated with gas torches. The welding process uses a steel canister (lined with refractory ceramic) containing a mixture of iron oxide and aluminum powders. This is positioned on top of the ceramic mold, and the powder is ignited. It reacts according to the equation

(16.2)Fe2O3+2Al=2Fe+Al2O3+850kJ.

The reaction heats the contents of the canister to ≈2500 °C, which melts the reduced iron. (The powders also contain pellets of alloying elements, so the molten metal is actually a steel, matching the composition of the rail steel). The molten steel then runs into the bottom of the mold cavity and rises up inside the gap between the rail ends. Some of the rail end is melted back, then the liquid steel solidifies to give a cast structure. Once the weld has gone solid, the mold is broken off, and the excess metal trimmed away. Finally, when the joint has cooled to room temperature, the profile of the running surface is accurately ground using a special rail mounted grinding wheel. The following clips show this actually being done (with varying degrees of safety precautions!).

http://www.youtube.com/watch?v=nR6K90cR8Lg

http://www.youtube.com/watch?v=unr_cbe335c&feature=related

http://www.youtube.com/watch?feature=endscreen&NR=1&v=foiY7X9zhF4

Obviously, the thermit process is also good for replacing short lengths of worn or cracked rail on site (these are cut out with an oxy-gas torch, a replacement length inserted into the gap, and the ends joined with thermit welds).

We are so used to thinking of welding in terms of electric arc welding that it is hard to realize that the thermit process was first used to make rail joints way back in 1899, before electric arc welding became popular. Although arc welding is a versatile joining method, we have seen that it has a number of potential disadvantages: metallurgical damage to parent material, distortion, hard to fully weld large cross sections, need for skilled workers, etc. Innovation often means exploring every possible joining method in order to achieve the optimum for the particular application, from friction welding lengths of oil pipe to electron beam welding thin diaphragms for aircraft pressure sensors http://www.camvaceng.com/aneroid-capsules.asp.

Examples

16.1

Copper capillary fittings are to be used to solder copper water pipes together as shown below:

The joint is designed so that the solder layer will yield in shear at the same axial load F that causes the main tube to fail by tensile yield. Estimate the required value of w, given the following data: t=1 mm; σy (copper)=120 MN m−2; σy (solder)=10 MN m−2.

Answer

24 mm.

16.2

A pieces of plain carbon steel containing 0.2 wt% carbon was case-carburized to give a case depth of 0.3 mm. The carburizing was done at a temperature of 1000 °C. The Fe–C phase diagram shows that, at this temperature, the iron can dissolve carbon to a maximum concentration of 1.4 wt%. Diffusion of carbon into the steel will almost immediately raise the level of carbon in the steel to a constant value of 1.4 wt% just beneath the surface of the steel. However, the concentration of carbon well below the surface will increase more slowly toward the maximum value of 1.4 wt% because of the time needed for the carbon to diffuse into the interior of the steel.

The diffusion of carbon into the steel is described by the time-dependent diffusion equation.

C(x,t)=(Cs−C0){1−erf(x2Dt)}+C0.

The symbols have the meanings: C, concentration of carbon at a distance x below the surface after time t; Cs, 1.4 wt% C; C0, 0.2 wt% C; D, diffusion coefficient for carbon in steel. The “error function,” erf (y), is given by

erf(y)=2π∫0ye−Z2dZ.

The following table gives values for this integral.

y 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
erf (y) 0 0.11 0.22 0.33 0.43 0.52 0.60 0.68
y 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 ∞
erf (y) 0.74 0.80 0.84 0.88 0.91 0.93 0.95 0.97 1.00

The diffusion coefficient may be taken as

D=9×10−6m2s−1exp{−125kJmol−1RT}
where R is the gas constant and T is the absolute temperature.

Calculate the time required for carburization, if the depth of the case is taken be the value of x for which C=0.5 wt% carbon.

Answer

8.8 min.

16.3

Using the equations and tabulated error function data from Example 16.2, show that the expression x=Dtgives the distance over which the concentration of the diffusion profile halves.

16.4

Outline the ways in which the surfaces of engineering materials may be treated to improve their wear resistance.

16.5

A bar of hot material (area A, length l) is subjected to a constant tensile load F along its axis. The material creeps under the load according to the equation

ε˙=Bσn,
where σ (=F/A) is the stress and B and n are constants. The bar contains a length of slightly reduced cross-sectional area. Show how the value of n affects the tendency of the bar to develop necks as it creeps. (Hints: The volume of the bar, Al=constant; differentiate this to show that dA/A=–dl/l=–dε; hence, show that A˙/A=–B(F/A)n=constant/An.)
16.6

Refer to the CCT diagram in Figure 16.2. Long cylindrical bars of this steel having diameters of 10, 50, and 200 mm are austenitized at 900 °C, then quenched into cold oil. For each bar, estimate the proportion of the various phases present in the as-quenched condition, and estimate the as-quenched Vickers hardness.

Answers

10 mm: 100% M, 460; 50 mm: 85% B, 15% M, 340; 200 mm: 100% B, 310.

16.7

Referring to Figure 16.2, explain the difference in the diameter scales for bars quenched in air, oil, and water.

16.8

Explain briefly what is meant by hardenability. Referring to the table in the data section at the end of the chapter, arrange the following steels in order of increasing hardenability: 0.40 C nickel steel; 0.40 C carbon steel; 0.40 C nickel–chromium steel; 0.40 C chromium–molybdenum steel; 0.39 C chromium steel.

16.9

The lower half of a forming die (for cold pressing an automotive subframe from thin sheet) has overall external dimensions of 1 m×1 m×0.2 m. Calculate the equivalent diameter, De for the die. Why is your value on the conservative side? Choose steels from the table in the data section which would give the die a fully martensitic structure after an oil quench. You are now told that the as-quenched hardness of the die must be 9 GN m−2. How much carbon should you have in the steel to achieve this? What would this amount of carbon do to the hardenability of the steels you identified earlier?

16.10

What are the advantages of friction welding compared to arc welding for joining lengths of thick-walled drill pipes?

16.11

What are the advantages of friction stir welding compared to arc welding for joining plates of work-hardened aluminum alloys?

16.12

Make a list of the devices you could envisage which utilize the special properties of SMAs.

16.13

What are the advantages of thermit welding compared to arc welding for joining railroad rails?

16.14

Give examples of how and why anaerobic adhesives have replaced traditional methods of joining in mechanical engineering.

16.15

Explain why the processes which take place during machining cause heating and wear of the cutting tool. How does this affect the choice of tool materials?

Heat treating steels

The addition of the alloying elements C, Mn, Cr, Mo, and Ni decreases the CCR for the formation of martensite and increases the size of section that can be quenched to give a fully martensitic structure. The following table gives approximate values for the maximum diameter of round bar which can be quenched in oil to give 100% martensite. The table lists typical ranges of compositions for carbon, carbon–manganese, and low-alloy steels as used in engineering applications. The concentrations of the alloying elements are given as weight %.

Steel Type C Si Mn Cr Mo Ni Diameter (mm)
C 0.13 – 0.60 – – – –
0.25 0.20 0.70 – – – 5
0.40 0.20 0.70 – – – 10
0.86 0.20 0.60 – – – 15
C–Mn 0.19 0.20 1.20 – – – 4
0.28 0.20 1.20 – – – 8
0.28 0.20 1.50 – – – 11
0.36 0.20 1.20 – – – 13
0.38 0.25 1.80 – – – 30
Ni 0.10 0.26 0.53 – – 3.65 7
0.16 0.25 0.60 0.20 – 1.50 15
0.10 0.20 0.40 – – 4.8 20
0.09 0.25 0.45 0.10 – 9.00 60
0.40 0.26 0.62 0.23 0.10 3.45 60
Ni–Cr 0.16 0.20 0.80 0.85 – 1.15 8
0.16 0.31 0.50 1.95 – 2.02 25
0.40 0.23 0.75 0.65 – 1.30 25
0.15 0.15 0.40 1.15 – 4.10 40
0.30 0.20 0.50 1.25 – 4.10 300
Cr 0.20 0.30 0.75 0.95 – – 8
0.38 0.25 0.70 0.50 – – 20
0.39 0.20 0.70 1.05 – – 30
0.59 0.25 0.60 0.65 – 0.20 40
0.24 0.37 0.27 13.3 – 0.32 500
Cr–Mo 0.14 0.25 0.55 0.60 0.55 – 5
0.12 0.30 0.45 0.85 0.60 0.16 8
0.27 0.13 0.60 0.74 0.55 0.19 20
0.40 0.20 0.85 1.05 0.30 – 50
0.32 0.25 0.55 3.05 0.40 0.30 110

The data have been obtained from CCT diagrams. The diagrams show significant variability even between steels of very similar composition. This is because the CCR is affected by the prior thermal and mechanical history of the steel. The data should be used only as a rough guide to tell us whether a quenched component of a particular size is likely to be fully martensitic.

Shape factors

See Equation (16.1). The f values given here are first order fits to more complex relationships obtained from finite element heat transfer modeling. They are therefore approximate only.

Infinitely long bar of rectangular cross section: thickness t, width b (0≤t/b≤1)

De=f×t,f≈1.7–0.6(t/b).

Circular bar length l, diameter d with axial hole diameter ϕ.

De=f×d.
(a)

ϕ/d=0:f≈1(0.75≤l/d≤∞),f≈1.33l/d(0≤l/d≤0.75).

(b)

ϕ/d=0.2:f≈0.6(0.45≤l/d≤∞),f≈1.33l/d(0≤l/d≤0.45).

(c)

ϕ/d=0.5:f≈0.4(0.3≤l/d≤∞),f≈1.33l/d(0≤l/d≤0.3).

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Carbon and ferritic alloy steels

J.F. Lancaster, in Metallurgy of Welding (Sixth Edition), 1999

8.7.2 Carbon steel for structural applications

Broadly speaking, unalloyed steels used in structural work (bridges, buildings, structures and shipbuilding) fall into two categories: low-carbon steel (up to 0.25% C) in the form of plates and sections, used for welded constructions, and higher-carbon steel (0.5–0.7% C) for reinforcing bar and rails. The second category constitutes about one-third of the total tonnage in the UK although this figure varies from time to time. Rails are joined by flash-butt welding into transportable lengths and field joints are made with low hydrogen coated electrodes or by thermit welding. Thermit welding is also used for joints that do not lend themselves to flash-butt welding, for example at points and on bends.

Reinforcing bar is welded with low hydrogen electrodes and with a preheat of 100–250 °C. Guidance as to procedure is given in AWS Standard D12-1.

Plain carbon steel is the preferred material for the bulk (about 90%) of structural work. Higher-tensile grades are required when stress is a governing consideration – for example, at the node sections of offshore constructions – but where deflection is the limiting factor increased tensile strength offers no advantage. The same applies to welded parts where fatigue loading is the main design consideration.

Both carbon and higher-tensile structural steels are specified in BS 4360 and in numbers of ASTM specifications, some of which are grouped according to applications, such as ASTM A709 for bridging. BS 4360 was replaced in part by the European Standard EN 10 025, which covers unalloyed steel having tensile strengths which range from 310 N mm− 2 to 690 N mm− 2 and which was published in 1990. Table 8.5 lists the specified compositions and properties of typical EN 10 025 grades, while Table 8.6 is a similar listing of those BS 4360 grades that have not been superseded. The mechanical properties are for sections up to 150 mm thick.

Table 8.5. Selected grades of structural steel to European Standard EN 10025

Grade Type of deoxidation* Chemical composition (max %) Ultimate stress (N mm− 2) Yield stre s (N mm− 2)s Elongation Charpy V impact
C Mn Si P s N Temp (°C) J
Fe 360 B FU 0.23 0.055 0.055 0.011 340–470 195 22 20 27
Fe 360 D2 FF 0.19 0.045 0.045 − 340–470 195 22 − 20 27
FE 430 B FN 0.25 0.055 0.055 0.011 400–540 225 18 20 27
Fe 430 D2 FF 0.21 0.045 0.045 − 400–540 225 18 − 20 27
Fe 510 B FN 0.27 1.7 0.6 0.055 0.055 0.011 470–630 295 18 20 27
Fe 510 D2 FF 0.24 1.7 0.6 0.045 0.045 − 470–630 295 18 − 20 27
Fe 510 DD FF 0.24 1.7 0.6 0.045 0.045 − 470–630 295 18 − 20 40
*
FU = rimming steel; FN = any other than rimming steel; FF = fully killed.

Table 8.6. Structural steel to BS 4360: 1990

Grade osition (%) Chemical composition (%) Ultimate strength (N mm− 2 Yield strength Elongation(%) Charpy V impact Supply condition*
Cmax Si Mnmax Pmax Smax Nb V Thickness limit (mm) N mm− 2 Temp (°C) J
40 EE 0.16 0.1–0.5 1.5 0.04 0.03 – – 340–500 150 205 25 –50 27 N
43 EE 0.16 0.1–0.5 1.5 0.04 0.03 – – 430–580 150 225 23 –50 27 N
50 EE 0.18 0.1–0.5 1.5 0.04 0.03 0.003–0.1 0.003–0.1 490–640 150 305 20 –50 27 N
50 F 0.16 0.1–0.5 1.5 0.025 0.025 0.003–0.08 0.003–0.1 490–640 40 390 20 –60 27 Q&T
55 C 0.22 0.6 max 1.6 0.04 0.04 0.003–0.1 0.003–0.2 550–700 25 430 19 0 27 AR or N
55 EE 0.22 0.1–0.5 1.6 0.04 0.03 0.003–0.1 0.003–0.2 550–700 63 400 19 –50 27 N
55 F 0.16 0.1–0.5 1.5 0.025 0.025 0.003–0.08 0.03/0.1 550–700 40 415 19 –60 27 Q&T
*
AR = as rolled; N = normalized; Q & T = quenched and tempered.

It is characteristic of structural steel that tensile and other tests are carried out on samples that represent a cast or batch, whereas in steel for boilers and pressure vessels tests are made on each plate.

Other standards cover through-thickness properties and weathering resistance. BS 6870 specifies three acceptance classes for through-thickness ductility: Z15, Z25 and Z35, where the number represents the minimum average percentage reduction of area for three transverse tests. Steels with such properties are marketed commercially as Hyzed steels.

Weathering steels are used for bridges and steel-framed buildings, particularly in the USA and to a lesser extent in the UK. These steels contain a small amount of copper and sometimes chromium, and when exposed to moderate atmospheric conditions develop a protective layer of rust on the surface. They have the advantage of reduced maintenance, but the rusty appearance is not universally acceptable.

Preheat requirements for non-alloyed structural steels (and this term includes microalloyed and controlled rolled plate) are specified in the UK in BS 5135. In this document four variables are used to determine the preheat: the hydrogen content of the weld deposit, the carbon equivalent of the steel, the combined thickness of the joint, and the heat input rate in kJmm− 1 The categories of hydrogen content are in accordance with the IIW recommendations (Section 8.5.3), likewise the carbon equivalent (equation 8.6). The combined thickness is the sum of the thickness of plate being joined by the weld. Based on these variables, graphs or tabulations give a minimum preheat and interpass temperature.

The AWS Code Dl.l for structural welding has a simpler approach to the problem. Here the variables are ASTM standard and grade, welding process and plate thickness. Coated electrodes are divided into two categories: low hydrogen and others. The required preheat is tabulated as a function of these variables. For example, the preheat for plain carbon steel up to 30ksi ultimate strength and thickness less than 19 mm (3/4 in) is nil, and for thicknesses between 19 and 38 mm, 66 °C, and so forth.

Preheating is an onerous and costly requirement in welding large structures, so the processes and procedures that reduce or eliminate preheat requirements are much favoured.

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Material evaluation and process selection

Peter Scallan, in Process Planning, 2003

General classification of joining processes

The other manufacturing processes considered so far have focused on the formation of specific geometries. Generally, they make a component from one material. However, there are occasions when a part may have to be made from two different materials. Thus, some sort of joining process must be employed. Joining processes can be categorized into three distinct types of process, namely welding, adhesive bonding and mechanical fastening as illustrated in Fig. 4.13.

Figure 4.13. General classification of joining processes

There is such a variation of processes that can be classified as welding, that this can be broken down further into fusion welding, solid state welding and brazing and soldering. Fusion welding can be defined as the melting together and coalescing of materials through the application of heat (Kalpakjian, 1995). The heat source can be further classified as either chemical or electrical. Chemical fusion welding covers welding processes that use a flame to apply the heat, for example, oxyfuel gas and thermit welding. Electrical fusion welding includes welding processes such as arc welding, resistance welding, electron beam welding and laser beam welding.

Brazing and soldering are also classified as welding processes. Brazing and soldering allow the joining of metals at lower temperatures. The other major difference from other welding processes is that a filler metal is used to join the two materials. The filler metal usually has a significantly lower melting point than the parent metals and thus they do not melt. The final welding category is that of solid state welding. The principle of solid state welding is that if two metals whose surfaces are free from contamination are forced together with a great enough force they will form a joint. In some solid state welding processes, such as cold pressure welding, no heat is applied but is generated during the process. However, in most processes heat is applied to improve the bonding between surfaces. This type of welding process can be classified as either electrical, chemical or mechanical solid state welding.

At its most basic, adhesive bonding is referring to the gluing together of materials. However, in recent years the use of adhesive bonding has increased due to the development of structural adhesives. Many disparate materials can be joined using adhesives, which has also increased the use of composite materials. Commonly used adhesives include epoxies, acrylics, urethanes and silicones.

With mechanical fasteners, neither fusion nor adhesion of the joining surfaces takes places. The most common type of mechanical fastener is the threaded fastener. This category includes screws, nuts and bolts. These tend to be bought-in items and should therefore be of a known quality and reliability. Non-threaded mechanical fasteners, including rivets, pins, retaining rings and staples, are also commonly used. Finally, there are a number of integral mechanical fasteners. These are generally interlocking tabs or seams on the joining parts themselves.

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