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

1. Preface

With the development of construction machinery towards large-scale and lightweight design, the demand for high-performance steel in construction machinery is increasing. As a result, the proportion of high-strength steel with a yield strength of 890MPa and above in structural steel is rising.

Q890D steel is a low-alloy, high-strength steel with a yield strength of ≥ 890MPa. This steel possesses high strength, high hardenability, and strong sensitivity to welding cold cracks. Among these characteristics, welding cold cracks pose a significant challenge in the application of Q890D steel.

To prevent the occurrence of welding cold cracks, technological measures such as preheating before welding have been studied and formulated. However, high-strength steel for construction machinery comes in various plate thicknesses and weld combinations.

The complex structure, large size, high welding workload, and poor accessibility of construction machinery make it difficult to apply excessively conservative welding preheating measures. Doing so would lead to issues such as low production efficiency, high labor intensity, energy consumption, and high costs.

Studying the welding process of Q890D high-strength steel with different thicknesses without preheating or low-temperature preheating is, therefore, significant in guiding the welding production of high-strength steel structural parts for construction machinery.

This post conducts oblique Y-groove tests and fillet weld welding tests that simulate the working conditions of products on Q890D steel plates with varying plate thicknesses. The aim is to determine the welding scheme without preheating and low-temperature preheating and provide a reference for the non-preheating welding process of high-strength steel.

2. Test plan

2.1 Test materials

The research object selected is the Q890D high-strength steel plate, which is produced by a steel plant and has a yield strength of ≥ 890 MPa. The plate has three different thicknesses: 10mm, 15mm, and 20mm.

The chemical composition and mechanical properties of the steel plate can be found in Table 1 and Table 2, respectively. For the welding process, a low matching solid core wire for gas shielded welding with a diameter of 1.2mm and a brand of ER76-G is used.

The chemical composition and mechanical properties of the deposited metal can be found in Table 1 and Table 3, respectively. The welding process employs GMAW with a shielding gas consisting of 80% Ar and 20% CO2.

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

Table 2 mechanical properties of Q890D steel

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

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 conducted.

The test weld is preheated at temperatures of 10°C and 100°C, respectively. Two groups of test welds are welded to primarily evaluate the cold crack sensitivity of the root pass of thick plate multi-layer welding.

The test scheme for the oblique Y groove is illustrated in Figure 1.

Fig. 1 inclined Y groove test

(1) Test parameters

The fusion weld is made using the same welding wire as the test weld.

To ensure the welding quality and avoid defects such as angular deformation, incomplete penetration, and cracks, the fusion weld uses double-sided welding and temperature control between passes.

The backing weld of the restrained weld is preheated to 150℃, with a welding current of 160A, arc voltage of 19V, and welding speed of 30cm/min. The interlayer temperature of the filler weld and cover weld is also maintained at 150℃, with a welding current of 250A, arc voltage of 27V, and welding speed of 40cm/min.

After the restrained weld has cooled down completely, a single pass test weld is carried out with complete restraint at both ends to evaluate the likelihood of cracks.

The test weld is performed in the flat welding position, with test temperatures of 10℃ and 100℃. Refer to Table 4 for the welding parameters used in the test weld.

Table 4 welding parameters of test weld

(2) Crack detection

The welded specimen must undergo natural air-cooling after welding.

After being left undisturbed for 48 hours, a visual inspection using a 20x magnifying glass must be conducted to detect any surface cracks. The crack rate of the surface must also be calculated.

The test weld, which has a length of approximately 70mm from the start of the weld width to the center of the weld crater, should be divided into 4 equal parts. Four metallographic samples (refer to Fig. 2) must be taken from each of the four parts.

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

It is generally believed that when the surface crack rate of a joint is less than 20%, it is safe to use it in production; however, there should be no root cracks.

Due to the welding production mode, the welded joint is in an unstable fixed state and experiences considerable stress concentration. Therefore, in order to ensure that there are no cracks, an acceptance requirement of a crack rate close to 0 is adopted.

2.3 Fillet weld process test

To better simulate the actual production conditions of our product, we have chosen to focus on the fillet weld of three plate thicknesses commonly used in our product structure. We will conduct welding tests and hardness tests on T-joints, using welding parameters that are commonly used in our production process, and taking into consideration actual working conditions such as the number of welding passes and interlayer temperature.

(1) Welding test

This test utilizes three commonly used plate thicknesses and welding forms in product structures, namely 20mm, 15mm, and 10mm.

The vertical plate and bottom plate have dimensions of 300mm × 150mm, with a 35° groove and a 1mm blunt edge on one side of the vertical plate, as depicted in Fig. 3.

As the actual product has a large structural size and long weld seam, the interlayer temperature during the test is only 70~80℃.

Hence, the test is conducted without preheating, and an interlayer temperature of around 70℃ is maintained.

Refer to Table 5 for the welding parameters of the fillet weld test.

Table 5 process parameters of fillet weld test

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 measure the hardness of the weld and its vicinity. The test location is indicated in Fig. 4.

Ensure that the test is conducted in compliance with the GB/T 2654-2008 test method for hardness of welded joints.

Fig. 4 hardness test position of fillet weld

3. Test results and analysis

3.1 Inclined Y groove test

After 48 hours of natural cooling following welding, the test weld surface should undergo a visual inspection (refer to Fig. 5).

Then, cut the test weld into four cross-sectional samples and perform a metallographic test. Also, conduct a crack detection under a 50x optical microscope (refer to Fig. 6).

The test results are presented in Table 6.

Fig. 5 inclined Y-groove test plate

a) Metallographic specimen

b) Crack detection

Fig. 6 crack detection of metallographic sample of test weld

Table 6 test results of inclined Y groove

Table 6 shows that when a 20mm plate is welded at room temperature and preheated at 100 ℃, both the surface crack rate and section crack rate are 0.

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

Under the same welding parameters, the thicker the steel plate, the lower the preheating temperature, and the faster the cooling rate after welding. This leads to more hardened structures forming in the heat affected zone of the weld, and a greater tendency for cold cracking. As a result, without preheating, 20mm plates are prone to cracking in the heat affected zone at the root of the weld.

Preheating can slow down the cooling rate after welding, resulting in a lower hardening and cold cracking tendency in the heat affected zone.

The tests demonstrate that 15mm and 10mm thick Q890D steel have good cold crack resistance, and preheating is not necessary. However, 20mm thick Q890D steel has poor cold crack resistance, and preheating to 100 ℃ can prevent cold cracking.

3.2 Fillet weld process test

Figure 7 displays the fillet weld joints of three test plates with thicknesses of 20mm, 15mm, and 10mm.

Following the cooling of the fillet weld test, two metallographic samples are cut in the middle of the weld using wire. The samples undergo rough grinding, fine grinding, sectioning, and corrosion before the microhardness tester is utilized to assess the hardness of the weld and its surrounding area in compliance with GB/T 2654-2008, the “hardness test method for welded joints.”

The test results are shown in Fig. 8.

Fig. 7 T-joint fillet weld test plate

Fig. 8 micro hardness test of fillet weld

The hardenability of steel is a significant factor in the formation of cold cracks during welding. Due to the welding thermal cycle, the heat-affected zone (HAZ) has a high tendency to harden, making it an area highly susceptible to cold cracking during welding.

Hardness and strength are closely related. In most cases, the higher the hardness, the greater the strength of the material, but it also leads to reduced plasticity and toughness.

Therefore, important welded structures must have certain restrictions on the maximum hardness of the HAZ of the welded joint.

Figure 8 shows that for T-joints with plate thicknesses of 20mm and 15mm without preheating, the HAZ has two allowable hardness values for materials with test points > 400HV10, whereas the maximum hardness value for 10mm plates in the HAZ is lower than 400HV10.

Moreover, under similar welding parameters and environmental conditions, the thicker the steel plate, the faster the cooling rate after welding, and the greater the tendency for hardening in the HAZ. This results in higher hardness values and decreased toughness.

If the hardness value exceeds the recommended allowable hardness value for 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 thicknesses of 20mm and 15mm without preheating during welding.

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

4. Conclusion

In this paper, oblique Y-groove tests and fillet weld tests were carried out to simulate the working conditions of Q890D steel plates with different thicknesses, and the following conclusions were drawn:

1. Welding crack tests were conducted on inclined Y-grooves.

The results showed that a 20mm thick plate would produce cracks without preheating at room temperature but would not produce cracks after preheating to 100℃. However, 15mm and 10mm thick plates did not produce cracks when welded at room temperature.

2. Fillet weld welding tests and hardness tests were carried out to simulate the actual production conditions of the product.

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

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

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