Handheld Laser Welding Machine

Handheld Laser Welding Machine

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Automatic Laser Welding Machine

Robotic Laser Welding Machine

Robotic Laser Welding Machine

Everything You Should Know About Welding Preheating

Preheating itself is often considered quite common.

It refers to heating the workpiece to be welded to above room temperature before or during welding.

Current specifications usually require several preheating temperature grades according to the applicable standard of materials.

The necessity, benefits and consequences of improper preheating will be illustrated by examples.

Welding Preheating

1. Techniques

Preheating refers to heating the workpiece to be welded above room temperature before welding or welding.

Pre-welding and post-welding specifications require preheating. However, under some conditions, other methods of preheating can also be used.

Whether preheating is required or not, preheating has the following advantages:

1. Reduce the shrinkage stress between the weld and the adjacent base metal, which is especially applicable to the weld with high-stress value.

2. Slow down the cooling rate in the key temperature range during weld cooling, prevent excessive hardening and reduce the ductility of weld and heat-affected zone (HAZ).

3. In the temperature range of 400 °F, slow down the cooling rate to allow more time for hydrogen to escape from the weld and adjacent base metal, so as to avoid hydrogen-induced cracks.

4. Remove contaminants.

The amount of preheating is not determined by the minimum standard of the specification, but by one or more of the following methods:

1. Calculation table

2. Carbon equivalent estimation

3. Crack parameter estimation

4. Spark test estimation

5. Rule of thumb

The preheating temperature range is usually suitable for various weld groove sizes and constraints.

Although the minimum preheating temperature is specified in many specifications, a lower preheating temperature will be used in some cases and a higher preheating temperature will be used in others.

2. Calculation sheet

There have always been various “preheating calculation tables” available.

Many “preheating calculation tables” adopt the form of linear or circular ruler to predict the preheating temperature through the identification of the material and thickness of the base metal.

3. Carbon equivalent

Carbon equivalent (CE) is a way to determine whether preheating is required and to what extent.

If CE ≤ 0.45%, preheating can be selected arbitrarily

If 0.45 = < CE < = 0.60%, the preheating temperature range is 200 °F to 400 °F (100 ° – 200 ° C)

If CE > 0.60%, the preheating temperature range is 400 ° to 700 °F (200 ° – 350 ° C)

When CE > 0.5, consider delaying the final nondestructive testing (NDE) for at least 24 hours to determine whether there are delayed cracks.

4. Crack parameters

Ito & bessyo parameter crack detection (PCM) can be used when the carbon equivalent is equal to or less than 0.17 wt -% or when high-strength steel is used.

This method can accurately predict when to preheat, when to force preheating and what temperature to preheat. Specifically

If PCM ≤ 0.15%, preheating is optional

If 0.15% < PCM < 0.26 – 0.28%, preheat to 200 ° – 400 °F (100 ° – 200 ° C).

If PCM > 0.26 – 0.28%, preheat to 400 ° – 700 °F (200 ° – 350 ° C).

5. Spark test

Spark test has been used for decades. It is a method to estimate the carbon content in carbon steel.

The higher the carbon content, the better the spark, and the more preheating is needed.

Although this method is not very accurate, it is simple.

The relative height of preheating temperature can be determined.

6. Rules of thumb

Another less precise but effective way to select the preheating temperature is to increase the preheating temperature by 100 °F (50 ° C) based on the carbon content (0.10 wt -%) at every 10 points.

For example, if the carbon content is 0.25 wt -%, the preheating temperature is 250 °F(125 ° C) or at least from 250 °F(125 ° C).

If coatings or other components exist near the weld, the preheating temperature determined according to the original production specification is not appropriate.

However, if the welding heat input is near the maximum range allowed by the standard process, the heat transferred to the welded components may be balanced by the welding heat input, causing the affected metal to be heated to or exceed the minimum value of preheating requirements.

Therefore, preheating can be more relaxed by external methods.

It should be noted that ranges and imprecise conversions (°F to ° C) are used here.

It was intentional. Preheating is not a very precise science.

In many cases, it is also normal to continuously increase the preheating temperature until the problem is solved (such as crack disappearance).

On the contrary, in some specific occasions, the purpose can be achieved even if the preheating temperature is lower than the recommended value or the temperature required by the specification.

7. Practical application

Attention should also be paid to the actual operation skills to avoid material softening caused by preheating.

Select welding processes and electrodes that rarely introduce hydrogen.

Some techniques can reduce or reduce residual stress.

Monitor carefully to ensure that the preheating method is used correctly.

The following descriptions are important for the successful use of these techniques.

8. Welding groove size and skills

The skills in the welding process have a great influence on the welding shrinkage, residual stress results, heat input control and avoiding cracks.

The longitudinal shrinkage of short weld is smaller than that of long weld.

Backhand welding or special welding sequences can also be used to reduce residual stresses.

Control or reduce heat input.

Linear welds with small oscillation can be used instead of large oscillation welds.

9. Reduce cracks

Craters and weld cracks can be reduced or eliminated by using appropriate manufacturing processes.

1) Compared with the weld with thin and wide section, the weld with circular section has the least cracks.

2) Avoid sudden start or stop welding. Welding operations and weld formation are controlled by up / down slope welding techniques or by electrical means of welding power supply.

3) There shall be enough deposited materials to avoid cracks caused by welding shrinkage or normal welding.

The experience to avoid cracks due to insufficient weld deposit material (which is also required in many production specifications) is that the amount of deposited metal is at least 3 ⁄ 8-in (10 mm) or 25% of the weld groove thickness.

10. Preheating method

In the workshop or in the field, flame heating (air-fuel or acetylene fuel), resistance heating, electronic induction heating and other methods can be used for preheating.

No matter what method is adopted, the preheating must be uniform.

Unless there are special requirements, the preheating shall penetrate the whole thickness of the weldment.

Fig. 1 shows the equipment using resistance (no insulation, later application) and induction heating.

resistance heating (left) and induction heating

Fig. 1 – resistance heating (left) and induction heating (right)

11. Preheating monitoring

Many devices can be used to measure and monitor the temperature.

The components or weldments to be welded shall be preheated until the heat completely saturates the material.

If possible, the degree of thermal penetration shall be tested or evaluated.

In general, for most welding applications, it is sufficient to monitor the temperature at a distance from the edge of the weld.

The welding groove must not be polluted by monitoring or reading temperature values.

12. Temperature indicator pen

These indicating pens or pencil-like tools melt at a certain temperature value, which can be used to simply and economically determine the minimum temperature reached by preheating, that is, the melting temperature of the indicating pen.

The disadvantage is that if the workpiece temperature is higher than the melting temperature of the indicator pen, it will not work.

When the workpiece temperature is too high, more indicating pens with different melting temperatures need to be used.

13. Electronic temperature monitoring

For preheating and welding operations, some direct measuring equipment such as contact pyrometers or direct reading thermocouples with analog or digital readings can also be used.

All measuring equipment shall be calibrated or their ability to measure the temperature range shall be verified in some way.

Because the thermocouple can continuously monitor and store data, it can use curve recorder or data acquisition system for preheating or PWHT operation.

AWS D10. 10 provides various schemes and examples of thermocouple placement positions.

14. “Indigenous law” monitoring

Many “indigenous methods” have been used for decades to determine whether the preheating temperature is sufficient.

Of course, one is to spray saliva or smoke directly on the workpiece.

The “sound” of saliva is the temperature indicator. Although not very accurate, many “veterans” use it.

Another more accurate way to determine the preheating temperature is to use an acetylene torch.

The flame is adjusted to high carbonization and gathers a layer of smoke gray in the area requiring preheating.

Then, adjust the welding torch to medium smoke and heat the smoke gray area.

When the smoke gray disappears, the surface temperature can reach more than 400 ° F (200 ° C).

Ensure that the preheating temperature is reached on the whole thickness of the workpiece and weldment area.

Most monitoring is only for the outer surface of the workpiece.

AWS D10. 10 the recommended practice is to provide useful guidance for the soaking zone. It is required that the whole workpiece thickness shall be heated during pipe welding.

Careful observation must be made during preheating to avoid overheating of preheated base metal, especially when resistance heating or induction heating method is applied.

Many shippers now require thermocouples to be placed under each resistance heating plate or induction coil assembly to monitor and avoid overheating.

15. Summary

No matter whether preheating is required or not and no matter what preheating method is used, preheating brings the following benefits:

Reducing the shrinkage stress of weld and adjacent base metal is especially beneficial to the welded joint with high constraint;

Slow down the cooling rate of the workpiece in the key temperature range, prevent excessive hardening of the workpiece and reduce the softening of weld and HAZ;

Slow down the cooling rate when the workpiece passes through the temperature range of 400 °F (200 ° C), so that hydrogen can diffuse from the weld and adjacent base metal more time, so as to prevent hydrogen-induced cracks;

Decontamination;

During preheating, it is best to heat the whole welding thickness evenly at the specified preheating temperature.

Too much local heating may cause material damage, which shall be avoided as far as possible.

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