· ·112

Position and function of preheating technology in welding application technology

Guanfu Sun 1, Weizhi Dai 1, 2, Wei Zhang 1

(1. Jinggong Steel Building Group, Zhejiang 312030, China; 2. China

Engineering Construction Welding Association, Beijing 100088, China)

Abstract: Combining with the superficial understanding of

preheating in practical engineering application, this paper expounds

the preheating technology starting with its definition. Considering

the current situation of insufficient understanding to preheating,

this paper makes explanation and illustration of relevant point,

introducing the related technical theory in simple terms. At the same

time, this paper also recommends several methods to determine the

preheating temperature, trying to make readers learn more about the

position and function of preheating technology in welding process by

combining with engineering project. t100, one of the most important

parameters, is introduced in this article, which has an important

role on the preventing of hydrogen-induced crack in strength steel

welding joint.

Key words: preheating; preheating process; secondary crystall-

ization; methods to determine

DOI: 10.7512/ j.issn.1001-2303.2017.13.07

Senior engineer; To be engaged in the development and research and application of steel structure welding application technology, and be good at the formulation of design and welding technology of large-scale complex steel structure system. There is a certain accomplishment in the welding technique of steel structure of construction steel. Strong theoretical foundation and rich practical experience.

Be willing to cooperate with international coun-terparts.

Weizhi Dai Email: DWZ470226@126.com

0 IntroductionThe high-required technical parameters of welding application is

always used in the steel structure and manufacturing industry. These

parameters are involved with basic theory and practical engineering

experience. In other words, they are the combining outcomes of the

basic theory and practice. For example, preheating is a very prominent

representation, which plays vital role in the steel structure welding

engineering. It can even be concluded that the properly application of

preheating technology determines the success or failure of welding

project to a certain extent!

However, it is regrettable of the point that the technical content

of preheating is low, which maybe because the preheating process

is too common superficially. As a result, there are few studies on

these parameters in welding academia and engineering, and the

articles related to preheating are rare as well, causing some fuzzy

understandings of the preheating technology.

In fact, though the preheating is widely used and universally known,

the related esoteric theory of welding applied technology is little-

known. So there exists a lot of confusions in engineering practice,

which inevitably produces incompetence in actual production. Thus, it

can be concluded that the welding quality must be pessimistic in these

confused and uncertain views.

1 Definition of preheating and relevant technical connotation(1) Definition of preheating.

Preheating is the process of heating the welding joints to a

temperature above the air temperature before welding. The process of

reheating during the welding process also belongs to the category of

preheating technology.

(2) Relevant technical connotation.

When the welding is completed, because of the dehydrogenation

heat treatment, the process of welded joint reheating ( including

cooling) is called postheat.

When the welding is completed, because of the relaxation of

welding residual stress, the process of welded joint reheating (including

cooling) is called post-weld heat treatment.

According to the above definition and relevant technical

connotation, although the high air temperature has certain advantages

to welding quality, it cannot substitute preheating, which is because that

the preheating temperature is much higher than the air temperature.

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Fig.1　Remote-infrared electric heating in thick

plate welding joint

Fig.2　Pipe preheating technique in thick plate welding joint

Preheating can control the weld cooling speed, reducing or avoiding

the generation of quenched martensite in the HAZ, and can make

the the hardness in the HAZ lower, helping hydrogen escape from

the welded joint. At the same time, preheating and can also reduce

welding stress. Preheating and accompanying preheating in the T-joint

and cruciform joint can effectively reduce the temperature difference

in the weld cooling process and prevent the production of hot crack.

Therefore, preheating is an effective measure to prevent the hydrogen

induced cracking and the hot cracking of the cruciform joint and T-joint

in the low-alloy high-strength steel welding.

The key of thick plate welding is to prevent welding cracks. Accurate

preheating, inter-pass temperature and postheat temperature are

the key to prevent crack, especially in the high strength steel welding,

which is because that the accurate control of preheating temperature,

inter-pass temperature and postheat temperature can directly affect

three elements of the hot crack generation in high strength steel

welding: diffusion hydrogen content, hardening tendency and

constraint stress.

Compared with the flame preheating method, the remote-infrared

electric heating has the advantage of accurate and reliable temperature

control and the ability to control the cooling rate. The most important

thing is: all welds adopted electric heating method are uniformly

heated, avoiding the produced additional stress in the uneven flame

preheating method, which effectively prevents the welding cracks, as

shown in Figure 1.

In some special occasions, preheating can be used in the pipe

heating, as shown in Figure 2. The pipe preheating technology has

advantages of low cost, uniform heating and easy heating operation.

While the drawbacks are difficult control of preheating temperature

and heating speed, low accuracy, etc.

2 Higher heating temperature is not always betterFor years, it was widely assumed that the results of NDT were

reliable, and that once the UT or RT tests were passed, everything was

all right. In fact, this is a very one-sided point and unreliable conclusion.

For example, when the preheating temperature and input energy

are high (high current, slow welding speed) in the welding process,

although UT and RT are all qualified, because of the coarse grains in

weld joint and HAZ, the comprehensive performance indicators reduce

sharply, and the impact toughness (αk) must be not qualified. This kind

of disqualification cannot be observed by naked eye directly. As a result,

there exist a lot of concealment, affecting the quality of the project.

In fact, the preheating temperature and welding heat input are

closely related to the secondary crystallization theory, which are direct

impacts of its vital parameters.

After the primary crystallization, the molten pool turn into solid,

going through a series of phase changes during the cooling process.

These phase change process are called the secondary crystallization of

weld metal, and the main effect parameters are t8/5, t8/3, t100, etc.

As shown in Figure 3:

t8/5: (cooling time of welding pool temperature from 800°C to 500°

t8/3: (cooling time of welding pool temperature from 800°C to 300°

t100: (cooling time of welding pool temperature from the highest to

100°C);

These three important technical parameters are not present in

the welding specification, but they are key indicators to the quality of

weld joint and important technical parameters in welding engineering

practice.

Fig.3 The important technical parameters in the welding thermal cycle

C);

C);

t100

t8/5

t8/3

T/°C800

500

300

100

t/s

· ·114

Fig.4　Transition node structure

Fig.5 Groove preheating

2.1 The main influence factors of t8/5(t8/3) and its impacts on the

welding joints performanceThe main influence factors of t8/5(t8/3) are: the thickness, type of

weld joint, welding heat input, preheating temperature, inter-pass

temperature, physical properties of base metal.

(1) t8/5(t8/3) has a great effect on the hardness of the welds.

The t8/5(t8/3) cooling time has a great effect on weld hardness,

and the hardness partly reflects the change of intensity. The longer of

t8/5(t8/3) cooling time, the lower of weld hardness.

(2) Effect of t8/5(t8/3) on impact property.

The extension of t8/5(t8/3) cooling time often leads to the reducing

of impact property and rise of HAZ impact brittleness temperature.

The strength also reduces, and the reduce degree depends on the type

and chemical composition of steel.

Unsuitable t8/5(t8/3) affects the overall performance of welded joints.

The extension of t8/5(t8/3) cooling time often leads to the reducing of

impact property and rise of HAZ impact brittleness temperature. The

strength also reduces, and the reduce degree depends on preheating

temperature and line energy. Unrestricted increase of the preheating

temperature must bring negative effects, so the higher heating

temperature is not always better.

It is obvious that in the actual welding engineering, appropriate

t8/5(t8/3) and ideal welding quality can be obtained by the controlling of

preheating temperature and welding heat input. It can be seen that the

preheating temperature and welding heat input play important role in

the welding thermal cycle.

2.2 Effect of t100

In the research of high strength steel welding cold cracking, it is

found that the cooling time of welding pool temperature from the

highest to 100°C has important effects on cold crack, so t100 is an

important parameter to cold crack tendency. There is no reliable formula

to calculate t100, which is mainly measured by means of trial.

It goes without saying that the preheating temperature and welding

heat input have a great effect on t100. Longer t100 is benefit to the

effusion of diffusion hydrogen hydrogen, eliminating and reducing

delay crack generation. But the extension of t100 must increase the

preheating temperature and welding heat input. However, the increase

of the welding heat input will bring coarse grains in weld and HAZ

and reduce the welding joint comprehensive performance. This is a

contradiction, and the method to solve it is adopting suitable preheating

temperature and welding heat input to obtain appropriate t8/5(t8/3),

and then conduct dehydrogenation postheat treatment (200°C~

350°C). In this way, the comprehensive performance of the welded

joint is guaranteed, and the t100 is extended to prevent the occurrence

of hydrogen induced cracks.

2.3 Preheating temperature monitoringWhen the heating temperature is determined, the temperature

should be monitored in real time to ensure the temperature is accurate.

Example: the preheating and post-thermal technology of an

engineering steel structure transition node and internal structure:

Because the steel structure transition node belongs to the ultra-wide

component, which is very difficult to make and transport. Therefore, in

the process of actual manufacturing, it is required to operate in each

section, and the concrete structure is shown in Figure 4.

3 Preheating and inter-pass temperature controlAccording to “Structural Welding Code-Steel” GB50661-2011:

preheating is required before welding process, and the welding spot

position and inter-pass temperature are control in the welding process,

as shown in table 1. The preheating range is 1.5 times plate thick and

not less than 100 mm. The temperature is measured by the remote

infrared thermometer at the reverse of plate with a distance of 50 mm

to weld center. The preheating method is flame heating, as shown in

Figure 5 and Figure 6.

Panel thickness Preheating temperature Inter-pass temperature

40≤δ＜60 80~100

120~23060≤δ＜80 100~120

δ≥80 120~130

Table 1　Preheating and inter-pass temperature

4 The determination of the preheating temperatureThis is a real problem that the engineering field demands prompt

solution, and it is a common concern of the welding industry.

There are a lot of scholars devoted to using calculation method

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Fig.6 Preheating, inter-pass temperature measurement and control

to determine welding preheating temperature, which has achieved

remarkable results and inspired people the essential knowledge of

preheat temperature, promoting the development and progress of

the theory research. However, the applications of these studies are too

complex and many uncertainties are demanded prompt solution, so

there is still a distance to the actual project application.

The authors recommend some methods for the determining of

preheating temperature:

4.1 Actual measurement methodThe minimum critical value t8/5 to avoid the cold crack is obtained

through the anti-cracking test, which uses Y-groove cracking test

or CTS anti-cracking test commonly. The impact toughness is

measured by actual plate welding test with varied heat input.

The upper limit can be determined by corresponding criterion

(normal temperature --40 °C, 27 J or 34 J ) , which can also be

converted or measured by welding heat input E. Y-groove cracking test

(cold cracking test) is mainly to assess the cold crack tendency of HAZ

and weld metal. The experiment was carried out in accordance with

the rules of “Testing Method of Y-groove Cracking Test” GB4975.1-84,

which is the most reliable method in engineering practice.

Example: the National Stadium “Bird's Nest” q460e-z35 Y-groove

cracking test.

The specimen plate is 110 mm thick, and the test condition is as

follows:

(1) SMAW.

① Welding rod: CHE557, electrode diameter φ4.0 mm.

② Type of welding machine: AOTAI inverter welding machine

ZX7-400STG.

(2) GMAW, FCAW-G.

① Welding wire: TM60, TWE -81k2, with wire diameter of 1.2 mm.

② Type of welding machine: AOTAI inverter welding machine

NBC-350.

③ Type of CO2: PRAXAIR (CO2≥99.9%, H2O≤50 ppm).

④ CO2 gas flow: 25 L/min.

Assembly requirements: test piece groove gap: 2±0.2 mm, as

shown in Figure7.

Fig.7 Shape and size of oblique Y groove cracking test

Preheating condition:

According to the steel highest hardness test results, the preheating

temperature of the first test are divided into 150 °C, 200 °C, 250 °C

three groups. At the same time, a 250 °C×2.5 h postheat specimens

group of CO2 arc welding under the condition of 250 °C preheating

is added. The preheating heating method is overall heat, and the

temperature of the test temperature must be kept at least two hours

after the required temperature is reached.

Check request:

At the end of the weld test, the surface crack examination was

carried out after 48 hours, and the section of each piece was treated

with bluing, observing the crack conditions.

The welding parameters and partial test results are shown in table 2.

The welding temperature can be determined according to table 2.

4.2 CCT diagram methodThe upper and lower limits of t8/5 can be estimated by the CCT curve

of base metal. However, the effect of HD (diffusion hydrogen) and RF

(constraint degree) should be considered when estimate the lower limit

of t8/5, and necessary correction is required.

For all kinds of low alloy steel, the weldability can be analyzed by its

own continuous cooling transformation (CCT) curve or stimulative HAZ

continuous cooling curve (SH-CCT). These curves can generally shows

microstructure and hardness under different welding thermal cycle

condition, and can estimate the cold crack tendency, so as to determine

the appropriate welding process.

Continuous cooling transformation curve (CCT curve) shows

the relationship of he beginning and finishing temperature, the start

and end time, the structural transformation, the hardness at room

temperature and the cooling speed in the weld and HAZ metal with

continuous cooling conditions. CCT curve is divided into weld metal

continuous cooling transformation curve (WM-CCT curve) and heat

affected zone of continuous cooling transformation curve (SH-CCT

curve). Because of the extensive application of SH-CCT curve in the

thermal affected area, the typical welding CCT curve often refers to the

SH-CCT curve.

The CCT curve is similar to the actual production condition, so it is

· ·116

Number Groove gap Preheating temperature Welding current Welding voltage Weld period Crack conditions Remark

C2 2.05 145 °C 170~180 24~25 29 no cracking ----

C6 1.9~2.2 149 °C 175~180 24~25 30 no cracking ----

C11 1.85~2.15 147 °C 170~180 23~25 32 no cracking ----

C3 1.8~2.0 199 °C 175~180 24~25 30 no cracking ----

C5 1.8~1.9 201 °C 175~180 24~25 31 no cracking ----

C7 2.03~2.19 203 °C 175~180 24~25 31 no cracking ----

C9 2.06~2.2 249 °C 170~180 24~25 30 no cracking ----

C10 1.99~2.2 252 °C 170~180 24~25 29 no cracking ----

C12 1.98~2.2 247 °C 175~180 24~25 31 no cracking ----

C1 1.8~1.85 Preheat 249 °C+ postheat 250 °C×2.5 h 170~180 24~25 30 no cracking ----

C4 1.9~2.2 Preheat 252 °C+ postheat 250 °C 175~180 24~25 29 no cracking ----

C8 2.03~2.2 Preheat 253 °C+ postheat 250 °C×2.5 h 175~180 24~25 30 no cracking ----

Table 2 Results of Y-groove cracking t results (CHE557) by manual arc welding

Table 3 Several common CE and Ceq formulas are used

Carbon equivalent formula Scope of application

International welding society (IIW) recommendation Steel: medium high strength (σb=500~900 MPa) non-adjustable low-alloy high-strength steel

chemical component: C≥0.18%

Japanese JIS standardSteel: low carbon quality low alloy high-strength steel

( σb = 500~1000 MPa )chemical component (mass fraction): C≤0.2%; Si≤0.55%; Mn≤1.5%; Cu≤0.5%; Ni≤2.5%; Cr≤1.25%; Mo≤0.7%; V≤0.1%; B≤0.006%

AWS recommendationsSteel: plain carbon steel and low alloy high-strength steel

chemical component (mass fraction):C＜0.2%; Mn＜1.5%; Ni＜3.3%; Cr＜1.0%;

Mo＜0.6%; Cu = 0.5%~1%; P = 0.05%~0.15%

CE (IIW)=C+ + + (%)Mn6

Ni+Cu15

Cr+Mo+V5

CE (JIS)=C+ + + + + + (%)Mn6

Si24

V14

Ni40

Cr5

Mo4

CE (AWS)=C+ + + + + + ( + ) (%)Mn6

Si24

Cu13

Ni15

Cr5

Mo4

P2

a useful reference for process planning. According to the CCT curve,

we can choose the most appropriate process specification, getting

ideal organization and improving the strength and plasticity to prevent

welding cracks, etc, which has important guiding significance for

establishing reasonable welding technology.

4.3 Carbon equivalent methodBased on the relationship of HAZ hardening and cold cracks tend,

and the steel chemical composition, the steel cold crack sensitivity can

be indirectly evaluated by chemical composition. The alloying elements

contents in steel (including C) converse into carbon quite content

according to its effect (assume the effect of C is 1), which can be

regarded as a rough evaluation of the steel cold cracking tendency. The

commonly used carbon equivalent formula are shown in table 3.

In the welding of alloyed reinforced steel, the carbon equivalent

method is used to determine the preheating temperature, which is

accurate and reliable. However, this method still has some shortages in

the case of microalloy reinforcement (high strength steel) welding.

4.4 Cold crack sensitivity index methodExcept for carbon equivalent, the hydrogen content and restraint

intensity of weld joint also have a great effect on the cold crack

tendency. So Japan expert ITO conducted Y-groove cracking test with

more than 200 different composition, thickness and weld hydrogen

content specimens, putting forward the chemical composition,

diffusion hydrogen and restraint intensity (or thickness) of cold crack

sensitivity index and other data (formula), which can determine the

welding preheating temperature of preventing the cold crack. Table 4

shows the data and calculation formula for determining the appropriate

preheating temperature.

Note: the postheat adopts furnace for heat treatment, using rock wool parcel the specimen after the thermal time is reached. 15.5 hours later,

the specimen is separated and the measured temperature is 130°C~135°C.

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Table 4 Cold crack sensitivity data and welding temperature determination

Cold crack sensitivity formula Preheat temperature calculation formula The application of the formula

To=1440Pc--392Y-groove cracking test-piece,

low alloy steel C≤0.17%[H]=1~5 ml/100g, δ=19~50 mm

To=1600PH--408Y-groove cracking test-piece,

low alloy steel C≤0.17%[H]>5ml/100g, R=500~3300 N/mm

To=1440PHT--330Y-groove cracking test-piece,

PHT considers the aggregation of hydrogen near the fusion zone

Pc = Pcm + + [H]60

δ600

Pw = Pcm + + +[H]60

δ600

R400000

PH = Pcm + 0.075 lg[H] + R400000

PHT = Pcm + 0.088 lg[λH'D] + R400000

notes: Pcm —— Cold crack sensitivity coefficient (%)

Type: δ —— Thickness (mm); H —— Hydrogen content in the weld (ml/100g)

Pcm = C+ + + + + + + +5B(%)Mn20

Cu20

V10

Ni60

Cr20

Mo15

Si30

This method is practical for the microalloy reinforcement steel. But

it is also affected by the diffusion hydrogen content and the availability

of quantitative test technology, so there are some shortcomings in the

application.

5 ConclusionForeign studies show that no matter whether the preheating is

required, or no matter what kind of preheating method is use, the

preheating technology can bring the following benefits. It can reduce

the shrinkage stress of weld and adjacent base metal, which is especially

beneficial for high constraint welded joint; it can slow down the cooling

rate of the critical temperature area, preventing excessive hardening

of the workpiece, and reducing the weld seam and HAZ softening;

In addition, it can slow down the cooling rate when the workpiece

passes through the temperature zone of 200℃, making the hydrogen

has more time to spread from the weld and adjacent base metal, and

preventing the generation of hydrogen induced cracking. The whole

welding thickness must be uniformly heated in the preheating process.

Local heating too much may cause material damage, which should be

avoided.

In addition, the measurements of adjusting welding parameters,

adopting the preheating multi-layer welding, inter-pass temperature

control, and other technological process can adjust and control the

welding thermal cycle, which can change the weldability of metal.

For example, when weld certain high-strength steel with hardened

inclinations, the material itself has certain cold crack sensitivity. If the

process is improperly selected, the welding joint may produce cold crack

or the plastic and toughness may be reduced. While the appropriate

filling material, reasonable welding thermal cycle, weld preheating,

post-heat treatment and other measures can get welding joint of well

performance with no crack defects.

Reference:[1]　Dai Weizhi, Liu Jingfeng, Gao Liang, Welding Engineering

Application Technology and Case of Construction Steel

Structure[M]. Beijing: Chemical Industry Press, 2016.