Process
Preheating is the heating of the workpiece to be welded to above room temperature before or during welding. Preheating is required by the specifications before and after welding. However, other methods may be used under certain conditions. Whether preheating is required or not, preheating has several advantages:
• Reduce shrinkage stresses in the weld and adjacent parent metal, which is especially applicable to welds with high-stress values.
• Slow the cooling rate during the critical temperature range during the cooling of the weld, preventing excessive hardening and reducing the ductility of the weld and heat-affected zone (HAZ).
• Slowing the cooling rate in the 400°F temperature range allows more time for hydrogen to escape from the weld and adjacent parent metal, avoiding hydrogen-induced cracking.
• Remove contaminants.
• The amount of preheat is not determined by the minimum standard of the specification, but by one or more of the following methods:
Calculation Tables
There have been a variety of “preheat calculation tables” available throughout history. Many of them are in the form of linear or circular slide rules and predict the preheat temperature by identifying the material and thickness of the parent metal.
Carbon Equivalent
Carbon equivalent (CE) is a way to determine if and to what extent preheating is required.
If CE ≤ 0.45%, preheating can be arbitrarily selected
If 0.45 =< CE <= 0.60%, preheating temperature range is 200°F to 400°F (100°–200°C)
If CE > 0.60%, preheating temperature range is 400° to 700°F (200°–350°C)
When CE > 0.5, consider delaying the final nondestructive examination (NDE) for at least 24 hours to determine if delayed cracking has occurred.
Crack Parameters
When the carbon equivalent is equal to or less than 0.17 wt-%, or when using high-strength steels, the Ito & Bessyo parameter crack detection (Pcm) can be used. This method can accurately predict when to preheat when to force preheating, and to what temperature. Specifically
If Pcm ≤ 0.15%, preheat at any temperature
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).
Spark Test
The spark test has been used for decades as a method of estimating the carbon content of carbon steel. The higher the carbon content, the better the spark and the more preheat is required. This method, while not very accurate, is a simple one. It can determine the relative high or low preheat temperature.
Rule of thumb
Another less accurate but effective method for selecting the preheat temperature is to calculate the preheat temperature increase by 100°F (50°C) for every 10 points of carbon content (0.10 wt-%). For example, if the carbon content is 0.25 wt-%, then the preheat temperature is 250°F (125°C) or at least starts at 250°F (125°C). If coatings or other components are present near the weld, then the preheat temperature determined by the original manufacturing specification is not appropriate. However, if the weld heat input is near the maximum range allowed by the standard process, the heat transferred to the welded components may be balanced by the weld heat input, causing the affected metal to be heated to or above the minimum preheat requirement so that a more relaxed preheat can be applied by external means. It is important to note that ranges and imprecise conversions (°F to°C) are used here. This is intentional. Preheating is not an exact science. In many cases, it is normal to continue to increase the preheat temperature until the problem is solved (such as crack disappearance). Conversely, in certain specific applications, even a lower preheat temperature than recommended or required by the specification can achieve the desired result.
Practical application
Practical skills must also be paid attention to avoid problems with preheating-induced material softening. Select welding processes and electrodes that introduce less hydrogen. Certain techniques can reduce or minimize residual stresses. Carefully monitor to ensure that the preheating method is used correctly. The following descriptions are important for the successful use of these techniques.
Welding groove size and techniques
The techniques used in the welding process have a great impact on the weld shrinkage of the workpiece, the residual stress results, the control of heat input, and the avoidance of cracks.
Short welds shrink less longitudinally than long welds. Residual stresses can also be reduced by using backhand welding or special welding sequences.
Control or reduce heat input. Linear welds with small oscillations can be used instead of large oscillating welds.
Reduce cracks
Arc craters and weld cracks can be reduced or eliminated by using proper manufacturing processes.
1) Welds with round cross-sections have the least cracking when welded compared to welds with thin, wide cross-sections.
2) Avoid sudden starts or stops. The welding operation and weld formation are controlled using up/down slope welding techniques or by electrical methods of the welding power source.
3) There should be enough deposited material to avoid cracking caused by weld shrinkage or normal welding. A rule of thumb to avoid cracking due to insufficient weld deposit (and required by many production specifications) is to deposit at least 3/8 in. (10 mm) or 25% of the weld groove thickness.
Preheating Methods
Preheating can be done in the shop or the field by flame heating (air-fuel or acetylene fuel), resistance heating, or electric induction heating. Regardless of the method used, preheating must be uniform and, unless otherwise required, through the entire thickness of the weld. Figure 1 shows equipment using resistance (uninsulated, as applied later) and induction heating.
Preheat Monitoring
A variety of equipment can be used to measure and monitor temperature. The assembly or weldment should be preheated to completely penetrate the material. If possible, the degree of heat penetration should be tested or evaluated. Generally, for most welding applications, monitoring the temperature at a distance from the edge of the weld is sufficient. Monitoring or reading the temperature value must not cause contamination of the weld groove.
Temperature Indicators
These pens or pencil-like tools melt at a certain temperature and can be used to easily and economically determine the minimum temperature to be reached during preheating, that is, the temperature at which the pen melts. The disadvantage is that it will not work if the workpiece temperature is above the melting temperature of the pen. When the workpiece temperature is too high, more pens with different melting temperatures are needed.
Electronic Temperature Monitoring
For preheating and welding operations, direct measurement devices such as contact pyrometers or direct reading thermocouples (with analog or digital readouts) can also be used. All measurement devices should be calibrated or have some method of verifying their ability to measure the temperature range. Because thermocouples can continuously monitor and store data, they can be used with curve recorders or data acquisition systems for preheating or PWHT operations. AWS D10.10 provides a variety of schemes and examples of thermocouple placement.
“Homegrown” Monitoring
Many “homegrown” methods have been used for decades to determine whether the preheat temperature is adequate. One, of course, is to spray spit or smoke liquid directly on the workpiece. The “snap” of the spit is the temperature indicator. Although not very accurate, many “old hands” use it.
Another more accurate method of determining preheat temperature is to use an acetylene torch. The flame is adjusted to high carbonization, which gathers a layer of soot in the area to be preheated. Then, the torch is adjusted to medium smoke, heating the soot area. When the soot disappears, the surface temperature can reach above 400°F (200°C).
Make sure that the preheat temperature is reached throughout the thickness of the workpiece and weldment area. Most monitoring is only on the outer surface of the workpiece. AWS D10.10 recommended practice provides a useful guide for soaking zones and requires that the full thickness of the workpiece be heated when welding pipe to pipe.
Careful observation must be made during preheating to avoid overheating the preheated base metal, especially when using resistance heating or induction heating methods. Many shippers now require thermocouples to be placed under each resistance heating plate or induction coil assembly to monitor and avoid overheating.
Summary
Whether preheating is required or not, and regardless of the preheating method used, preheating provides the following benefits: Reduce shrinkage stresses in the weld and adjacent parent metal, which is particularly beneficial for highly constrained weld joints; Slow down the cooling rate of the workpiece in the critical temperature range, preventing excessive hardening of the workpiece and reducing softening of the weld and HAZ; Slow down the cooling rate of the workpiece when passing through the 400°F (200°C) temperature range, allowing hydrogen more time to diffuse from the weld and adjacent parent metal, preventing hydrogen-induced cracking; Remove contamination; When preheating, it is best to heat the entire weld thickness evenly at the specified preheat temperature. Excessive local heating may cause material damage, so try to avoid it.
Post time: Nov-04-2024