Non Preheating Welding Cracks of Q890 High Strength Steel: What Should Be Done?

1. Preface

With the development of construction machinery in the direction of large-scale and lightweight, the requirements of steel for construction machinery for performance are higher and higher, and the proportion of high-strength steel with yield strength of 890MPa and above in structural steel is increasing.

Q890D steel is a low-alloy high-strength steel with yield strength ≥ 890MPa.

The steel has high strength, high hardenability and strong sensitivity to welding cold crack. Among them, welding cold crack is the key problem that needs to be solved first in its application process.

In order to prevent the generation of welding cold cracks, the technological measures of preheating before welding are studied and formulated.

However, there are many plate thicknesses and weld combinations of high-strength steel for construction machinery, coupled with the production characteristics of complex structure, large size, large welding workload and poor weld accessibility of construction machinery, too conservative welding preheating measures will produce problems such as low production efficiency, high labor intensity, energy consumption and high cost.

It can be seen that studying the welding process of Q890D high-strength steel with different thickness without preheating or low temperature preheating has important guiding significance for the welding production of high-strength steel structural parts of construction machinery.

In this post, the oblique Y groove test and the fillet weld welding test simulating the working condition of products are carried out on Q890D steel plates with different plate thickness, so as to determine the welding scheme without preheating and low temperature preheating, and provide reference for the non preheating welding process of high-strength steel.

2. Test plan

2.1 Test materials

Q890D high-strength steel plate with yield strength ≥ 890mpa produced by a steel plant is selected as the research object.

The plate thickness is 10mm, 15mm and 20mm respectively.

See Table 1 and table 2 for chemical composition and mechanical properties respectively.

The welding material is a low matching solid core wire for gas shielded welding, with the brand of ER76-G and the diameter of 1.2mm.

See Table 1 and table 3 for the chemical composition and mechanical properties of the deposited metal.

GMAW with 80% Ar + 20% CO2 is used.

Table 1 chemical composition (mass fraction) of Q890D steel and ER76-G welding wire %

Material ScienceCSiMnPSCrNbVTiMoNiCe
Q890D0.160.301.120.0090.0080.210.0220.0530.0190.540.020.50
ER76-G0.070.571.730.0060.0070.290.301.33

Table 2 mechanical properties of Q890D steel

Yield strength /mpaTensile strength /mpaElongation after fracture (%)Cold bending (180 °)– 20 ℃ impact absorbed energy /j
996102416.5d=4a, qualified172

Table 3 Mechanical Properties of ER76-G welding wire deposited metal

Yield strength MpaTensile strength /mpaElongation after fracture ()– 20 ℃ impact absorbed energy
75081520.0121

2.2 Inclined Y groove welding crack test

According to GB/T 32260.2-2015 destructive testing of welds in metallic materials – cold crack testing of weldments – arc welding methods – Part 2: self restraint test, the welding crack test of oblique Y groove is carried out.

The preheating temperature of the test weld is 10 ℃ and 100 ℃ respectively.

Two groups of test welds are welded, which are mainly used to evaluate the cold crack sensitivity of the root pass of thick plate multi-layer welding.

The oblique y groove test scheme is shown in Fig. 1.

Non Preheating Welding Cracks of Q890 High Strength Steel: What Should Be Done? 1

Fig. 1 inclined Y groove test

(1) Test parameters

The binding weld is the same as the welding wire of the test weld.

The binding weld adopts double-sided welding, preheating and controlling the temperature between passes to avoid the defects that affect the welding quality, such as angular deformation, incomplete penetration and cracks.

The preheating temperature of backing welding of restrained weld is 150 ℃, the welding current is 160A, the arc voltage is 19V, and the welding speed is 30cm/min;

The interlayer temperature of filler welding and cover welding is 150 ℃, the welding current is 250A, the arc voltage is 27V, and the welding speed is 40cm / min.

After the restrained weld is completely cooled, carry out single pass test weld welding under the condition of complete restraint at both ends to evaluate the crack tendency.

The test weld is welded at the flat welding position, and the test temperature is 10 ℃ and 100 ℃ respectively.

See Table 4 for the welding parameters of the test weld.

Table 4 welding parameters of test weld

Sample NoPlate thickness /mmPreheating temperature / ℃Welding current /AArc voltage /NWelding speed /cm*min-1Heat input /kj cm-1
LDI2010250274010.1
LD220100250274010.1
LD31510250274010.1
LD415100250274010.1
LD51010250274010.1
LD610100250274010.1

(2) Crack detection

The test weld shall be naturally air cooled after welding.

After being placed for 48h, visual inspection and 20x magnifying glass shall be used to observe and detect, and the surface crack rate shall be calculated.

Divide the test weld with a length of about 70mm between the beginning of the weld width and the center of the weld crater into 4 equal parts, and take 4 metallographic samples (see Fig. 2).

metallographic sampling of oblique y groove test

Fig. 2 metallographic sampling of oblique y groove test

Note: 1 refers to cutting along the width direction; 2 is the section position.

Observe the cracks in the weld metal and heat affected zone on the profile with a microscope of more than 50 times, and calculate the crack rate of the profile.

(3) Evaluation method

Due to the great restraint of the joint, it is generally believed that when the surface crack rate is less than 20%, it is safe to use it in production, but there should be no root crack.

Considering the welding production mode, the welded joint is in an unstable fixed state and has considerable stress concentration.

In order to ensure that there is no crack, the acceptance requirement of crack rate close to 0 is adopted.

2.3 Fillet weld process test

In order to further simulate the actual production conditions of the product, the fillet weld of three plate thicknesses commonly used in the product structure is taken as the research object, and the fillet weld welding test and hardness test of T-joint are carried out by using the welding parameters commonly used in production and the actual conditions such as the number of welding passes and interlayer temperature under the actual working conditions.

(1) Welding test

This test adopts three kinds of plate thickness and weld forms commonly used in product structures: 20mm, 15mm and 10mm.

The length and width of vertical plate and bottom plate are 300mm × 150mm, with 35 ° groove and 1mm blunt edge on one side of the vertical plate, as shown in Fig. 3.

Due to the large structural size and long weld seam of the actual product, the actual interlayer temperature is only 70~80 ℃.

Therefore, the test conditions without preheating and the interlayer temperature of about 70 ℃ are adopted for the test.

See Table 5 for welding parameters of fillet weld test.

Table 5 process parameters of fillet weld test

Sample No

Plate thickness (t1+t2) /mm

Weld bead

Preheating or interpass temperature / ℃

Welding current /A

Arc voltage /N

Welding speed /cmmin-1

Heat input /kj cm-1

LJI

20+20

Backing welding

room temperature

160

19

30

6.1

Fill welding 2~4 passes

70

250

27

40

10.1

Cover welding 5-6 passes

70

250

27

40

10.1

LJ2

15+15

Backing welding

room temperature

160

19

30

6.1

Fill welding 2~3 passes

70

250

27

40

10.1

Cover welding

70

250

27

40

10.1

LJ3

10+10

Backing welding

room temperature

160

19

30

6.1

Cover welding

70

250

27

40

10.1

T-joint fillet weld

Fig. 3 T-joint fillet weld

Note: δ is the plate thickness.

(2) Microhardness test

Take two metallographic samples from the middle of the fillet weld of the welding test.

After rough grinding, fine grinding, sectioning and corrosion, use a microhardness tester to test the hardness of the weld and its vicinity. The test location is shown in Fig. 4.

It shall be carried out in accordance with GB/T 2654-2008 test method for hardness of welded joints.

hardness test position of fillet weld

Fig. 4 hardness test position of fillet weld

3. Test results and analysis

3.1 Inclined Y groove test

After 48h natural cooling after welding, the test weld surface shall be visually inspected (see Fig. 5).

Then cut the test weld into 4 cross-section samples and carry out metallographic test, and carry out crack detection under a 50x optical microscope (see Fig. 6).

The test results are shown in Table 6.

inclined Y-groove test plate

Fig. 5 inclined Y-groove test plate

Metallographic specimen

a) Metallographic specimen

Crack detection

b) Crack detection

Fig. 6 crack detection of metallographic sample of test weld

Table 6 test results of inclined Y groove

Sample NoPlate thickness /mmPreheating temperature / ℃Surface crack rateSection crack rate (%)
LDI2010055
LD32010000
LD2151000
LD41510000
LD5101000
LD61010000

It can be seen from table 6 that the surface crack rate and section crack rate of 20mm plate are both 0 when welded at room temperature and preheated at 100 ℃.

The surface crack rate and section crack rate of 15mm thick and 10mm thick test plates are 0 whether they are at room temperature or preheated at 100 ℃.

Under the same welding parameters, the thicker the steel plate is, the lower the preheating temperature is, the faster the cooling rate after welding is, the more hardened structures are formed in the heat affected zone of the weld, and the greater the cold cracking tendency is. Therefore, 20mm plate is easy to crack in the heat affected zone at the root of the weld without preheating.

Preheating can slow down the cooling rate after welding, and the hardening tendency and cold cracking tendency of the heat affected zone are small.

The test shows that the cold crack resistance of 15mm and 10mm thick Q890D steel is good, and preheating welding is not necessary, while the cold crack resistance of 20mm thick Q890D steel is poor, and preheating to 100 ℃ can prevent cold cracks.

3.2 Fillet weld process test

The fillet weld joints of three kinds of test plates with thickness of 20mm, 15mm and 10mm are shown in Fig. 7.

After the fillet weld test is cooled, two metallographic samples are cut by wire in the middle of the weld.

After rough grinding, fine grinding, sectioning and corrosion, microhardness tester is used to detect the hardness of the weld and its vicinity in accordance with GB/T 2654-2008 “hardness test method for welded joints”.

The test results are shown in Fig. 8.

Non Preheating Welding Cracks of Q890 High Strength Steel: What Should Be Done? 2

Fig. 7 T-joint fillet weld test plate

micro hardness test of fillet weld

Fig. 8 micro hardness test of fillet weld

The hardenability of steel is one of the main reasons for the formation of welding cold cracks. Affected by the welding thermal cycle, the welding heat affected zone has a high hardening tendency, which is the area with high sensitivity to welding cold cracks.

There is a corresponding relationship between hardness and strength.

In most cases, the higher the hardness, the higher the strength of the corresponding material, and the worse the corresponding plasticity and toughness.

Therefore, in important welded structures, there should be certain restrictions on the maximum hardness of the heat affected zone of the welded joint.

It can be seen from fig. 8 that for T-joints with 20mm and 15mm plate thickness, without preheating, the hardness value of heat affected zone has two allowable hardness values of materials with test points > 400HV10, while the maximum hardness value of 10mm plate in heat affected zone is lower than 400HV10.

In addition, under the same welding parameters and environmental conditions, the thicker the steel plate, the faster the cooling rate after welding, the greater the hardening tendency of the heat affected zone, so that the hardness value is higher and the toughness decreases.

When the hardness value is higher than the allowable hardness value recommended by the material, there is a risk of welding cold cracks in the structural parts with high restraint.

Therefore, there is a certain risk of cold cracking for T-joints with plate thickness of 20mm and 15mm without preheating welding.

However, when the T-joint with a thickness of 10mm is welded without preheating, the tendency of hardening and cold cracking is low, and the safety is high.

4. Conclusion

In this paper, the oblique y groove test and the fillet weld test simulating the working condition of products are carried out for Q890D steel plates with different thickness, and the following conclusions are drawn:

1) Carry out the welding crack test of inclined y groove.

The results show that the 20mm thick plate will produce cracks without preheating at room temperature, and will not produce cracks after preheating to 100 ℃;

However, 15mm and 10mm thick plates will not produce cracks when welded at room temperature.

2) The fillet weld welding test and hardness test simulating the actual production conditions of the product were carried out.

The results showed that the samples with 20mm plate thickness and 15mm plate thickness had multiple recommended hardness values of materials with hardness values greater than 400HV10 in the heat affected zone, while all hardness values of 10mm plate thickness were lower than the recommended hardness values of materials.

3) Considering the oblique Y groove test and the fillet weld test simulating the working conditions of the product, preheating measures are not required for the welding of 10mm plates.

The minimum preheating temperature of 20mm plates is 100 ℃, and it is recommended to preheat 100 ℃ for 15mm plates in the key welds with large restraint, while it is not necessary to preheat in the non key welds with small restraint.

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