Ensuring Quality and Safety in Explosive Welding: Best Practices

Quality Inspection of Explosion Weld Joints

1.Common Welding Defects

Common defects in explosion weld joints include macro and micro defects, with the macro defects mainly being poor interface bonding, bulges, and large area melting.

(1) Poor Bonding

Poor bonding refers to the failure of the covering plate and the base plate to bond fully or mostly after explosion welding, or if they do bond, the bond strength is low.

To overcome these defects, you should first choose explosives with a lower detonation speed, then use an adequate amount of explosives and an appropriate gap distance.

Moreover, the initiation position should be chosen carefully to shorten the gap exhaust path and create favorable exhaust conditions.

(2) Bulging

Bulging is when there is a protrusion in a localized area on the composite plate, filled with gas, and produces a “bang” sound when hit. To eliminate bulges, in addition to choosing the right amount of explosives and spacing, you should also create good exhaust conditions.

(3) Large Area Melting

Large area melting often occurs in bimetallic plate explosion welding, resulting in large area melting on the bonding surface. To alleviate and eliminate this phenomenon, use low explosion speed explosives and center detonation method to create good exhaust conditions.

(4) Surface Burn

Surface burn refers to the cover plate being thermally oxidized and burnt by the explosion. Preventive measures include using low explosion speed explosives and placing a protective layer of asphalt, etc., between the explosive and the cover plate.

(5) Explosion Deformation

Explosion deformation refers to the macroscopic and irregular changes in the dimensions and shape of the composite plate in length, width, and thickness after explosion welding.

While it’s difficult to avoid this deformation under normal circumstances, you can take measures to alleviate it, such as increasing the rigidity of the base plate or adopting other technical measures. The deformed composite plate must be leveled or straightened before processing or use.

(6) Explosion Brittleness

Explosion brittleness is common in materials with low room temperature impact properties or very high strength or hardness. Unless thermal explosion welding processes are used (preheating the workpiece before explosion), it’s generally difficult to eliminate.

(7) Unbonded Detonator Area

The unbonded detonator area is the area where the detonator is initiated. It’s caused by insufficient energy and poor exhaust, causing the area to remain unbonded. It can be avoided by increasing the amount of explosives in this area or initiating it outside the composite area.

(8) Edge Rupture

Edge rupture is a phenomenon where the overlay layer of the composite plate’s periphery or the front end of the composite tube (rod) is damaged and ruptured due to boundary effects. The main reason for this phenomenon is excessive energy at the periphery or front end.

Therefore, by reducing the amount of explosives at the edge or front end, increasing the size of the overlay plate or tube, or grooving around the periphery outside the bonding surface of the thick plate, this phenomenon can be reduced or eliminated.

(9) Explosion Damage

Explosion damage occurs when the explosives clump together or are unevenly distributed, causing excessive local energy, or when there are solid hard objects mixed in the explosives, which hit the plate surface causing pockmarks, depressions, or small grooves.

Explosion damage affects surface quality, and preventive measures mainly include purifying and evenly distributing the explosives.

In addition to the above macro defects, microscopic defects such as micro-cracks, microscopic pores, inclusions, or coarse grain structures may be detected in the interior of the explosion-welded composite plate using non-destructive and destructive methods.

These microscopic defects can cause uneven microscopic structures in the explosion composite plate, affecting the mechanical properties of the composite plate.

2.Inspection Methods

Quality inspection methods for explosion weld joints include non-destructive and destructive inspections.

(1) Non-Destructive Inspection

1) Surface Quality Inspection.

The main purpose is to inspect the surface and appearance of the explosion-welded composite plate, such as damage, rupture, oxidation, and warping deformation.

2) Tapping Inspection.

Each position of the overlay is lightly tapped with a hammer, the bonding situation of the interface can be preliminarily judged based on the sound, and the bonding area rate can be roughly estimated.

3) Ultrasonic Inspection.

Use ultrasonic waves to detect the bonding situation of the interface and quantitatively measure the bonding area.

(2) Destructive Inspection

In accordance with GB/T 6396-2008, the bonding strength of the explosion-welded composite plate is determined by shear and bending tests, and its tensile strength is determined by tension tests.

In addition to the above methods, inspections such as microhardness, metallography, impact, torsion, fatigue, thermal cycling, and various corrosion resistance can be conducted on the explosion-welded composite parts, depending on the specific situation and needs.

Explosion Welding Safety and Protection

Explosion welding uses explosives as the energy source for welding, and there are many unsafe factors during the explosion process. Therefore, safety issues during the process of explosion welding are particularly important and a strict management system and

implementation procedures must be established. The safety issues that must be paid attention to in explosion welding practice are as follows:

1) The explosion site should be set up far from buildings. There should be no objects that could be damaged around the place where explosion welding is carried out.

2) The explosives depot must be strictly managed, with management personnel on duty day and night, and outsiders are not allowed in. Explosives, detonators, and detonating cords and other pyrotechnic items must be stored separately by category. The management of the depot and the outflow must be strictly controlled, and all related records must be kept.

3) Explosives and raw materials, detonators, and workers must be transported separately. It is strictly forbidden to transport explosives and detonators in the same vehicle.

4) All personnel engaged in explosion welding work must undergo vocational skill training and assessment. Only those who pass the assessment and obtain an operation certificate can carry out the operation.

5) All workers must be supervised by the safety and security department and comply with relevant national policies and laws.

The process of explosion welding should be coordinated and commanded by a dedicated person, according to pre-planned process procedures. Detonators and initiators should be managed by designated personnel.

6) Before carrying out explosion welding operations, ensure that all workers and spare items are in a safe zone and that all personnel have taken sound and shock protection measures.

Give a predetermined signal before detonation, and workers can return to the explosion site only 3 minutes after the explosion of the explosives. If the explosives fail to explode, inspection and handling should be carried out on-site only 3 minutes later. Workers are not allowed to bring flames or fire sources into the work site.

In explosion welding production, low-explosion-speed mixed explosives, such as ammonium salt and ammonium oil explosives, are typically used. The former is composed of ammonium nitrate and a certain proportion of salt, while the latter is composed of ammonium nitrate and a certain proportion of diesel, using only a small amount of TNT to detonate the explosives.

Ammonium nitrate is a common fertilizer and is very stable; its “inertness” is even greater after mixing with salt and diesel. Granular ammonium nitrate and flake TNT can be crushed into powder by a ball mill without exploding. Ammonium salt and ammonium oil explosives can stably explode only under the detonation of high-explosion-speed explosives such as TNT.

TNT explosives still need to be detonated by a detonator, and the high-speed explosives in the detonator only explode under a high voltage of hundreds of volts from the initiator. Therefore, in on-site operations, detonators and initiators must be strictly controlled to avoid safety accidents.

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