Selection of Filler Metal and Recommended Heat Input for Low Alloy Steel welding

Why to use low alloy steels?

Companies choose low-alloy steels because they have higher strength, toughness and/or performance under harsh operating conditions compared to carbon steels. However, the mechanical properties of low-alloy steel will vary greatly due to different alloying elements. With the increasing popularity of these materials in industry-from structural steel and pressure vessel fabricators to heavy machinery manufacturing-the welding electrode or filler materials and welding processes required for their connection have become more and more important. Remember the following points to eliminate serious welding problems during welding.

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What is low alloy steel?

low alloy steel have many different alloying elements. Among them, Ni (nickel), Mo (molybdenum) and Cr (chromium) usually account for more than 0.5% but less than 5% of the total alloying elements.

 Each element gives the material specific properties. For example, in addition to moderately increasing tensile strength, nickel can also improve low-temperature toughness. Molybdenum and chromium increase tensile strength and help maintain strength at elevated temperatures. Copper can be used in combination with other elements to improve atmospheric corrosion resistance, just like weathering steel.

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 The tensile strength of most low-alloy steels exceeds 70 ksi, and some even exceed 120 ksi. Some low-alloy steels have excellent weldability, while others are weldable depending on alloying. Remember, as the alloy content increases, the material tends to form a more brittle microstructure after welding, making it difficult to successfully weld.

Since each alloy element affects the characteristics and performance of the weld metal, it is very important to correctly match the filler material with the base metal.

 Choosing Filler Metals for Low Alloy Steel Welding

When selecting the correct filler metal for low alloy steel applications, it is important to determine the size and quality of the base metal; the easiest way to determine the minimum requirements is to consult the base material specification. Many ASTM specifications have requirements for chemical and mechanical properties, while many AISI/SAE specifications only have requirements for chemical properties and usually require minimum mechanical properties to be specified at the time of purchase.

Welding Electrode/ Filler wire/ Filler metal Selection Chart

Consider the chemical composition of base metals. The elements used to improve the properties of the substrate are also used in filler materials. When looking for filler metals that are close to the performance of the base metal, remember that the chemical composition of some low-alloy filler metal classifications will perfectly match the composition of the base metal. This is due to the significant difference between steel production and weld metal production. Since there is rarely an exact match, matching the nominal chemical composition requirements of the application-for example, the required level of corrosion resistance or high temperature performance-helps the filling material and the base material have similar characteristics.

 filler metal can also be adjusted by the mechanical properties of the substrate. Please note that certain alloys can be used under annealing, normalizing, or quenching and tempering conditions, depending on the processing technology, which may result in significant differences in the mechanical properties of the same material. For example, 4130 and 4140 chromium molybdenum steel.

When selecting a filler material that utilizes the mechanical properties of the substrate, the most common one is to adjust the tensile strength and/or yield point according to the application requirements. According to the AWS D1.1 or AWS D1.5, the classification of the welding electrode that meets the code requirements are  given in AWS D1.1 Table 3.1 in older edition and Table 6.9 in new edition of the code and AWS D1.5 Table 4.1.

 In some high-strength, low-alloy welded steel structures, it often happens that the tensile strength or yield point of the filler metal is lower than the base metal. As the tensile strength of the weld metal increases, the ductility tends to decrease. If the component is designed so that the stress is not concentrated in the weld, underfitting can be a way to maximize weld ductility and fatigue life.

Control & Check Hydrogen Level in Welding

What is Hydrogen Cracking or Delayed Cracking or Cold Cracking?

When selecting the filling material, make sure it has a low proportion of diffusible hydrogen. Since low alloy welds tend to form brittle microstructures, hydrogen cracks are more likely to occur.

 Filler materials with less diffusive hydrogen will reduce one of several hydrogen sources (others include base material and welding environment), thereby reducing the risk of hydrogen cracking. Look for filler metals with the H4 or H8 designator, the smaller the number in the identifier, the lower the diffusion hydrogen. Right storage conditions of low-alloy filler material also helps to minimize hydrogen absorption and subsequent contribution to the weld metal. Avoid drastic temperature changes when storing filler metals, minimize the total exposure time (out-of-package time), and always follow the manufacturer’s storage recommendations.

Using preheat temperatures above 212 degrees Fahrenheit and removing base metal rust, scale, or coatings (such as oil, grease, or scale) are other ways to reduce hydrogen in weld metal. Combined with properly stored low-hydrogen filler metals, these technologies help reduce the susceptibility to hydrogen cracking.

Take Note of the cooling rate

The cooling rate of the weld and the heat-affected zone (HAZ) is important because it affects the formation of the structure in these areas and the subsequent mechanical properties.

 The heat-affected zone is the area near the weld that is not melted by the arc, but it still undergoes microstructure changes due to processing heat. Because low-alloy steel has high hardenability, it is easier to form a brittle microstructure in HAZ. Therefore, HAZ performance is an important aspect of successful low-alloy steel welding.

 A cooling rate that is too fast is conducive to the formation of brittle microstructures, while a cooling rate that is too slow and excessive heat input will result in a very rough microstructure, which may not provide good final performance. Choosing an appropriate cooling rate balance behavior is the key.

 Consider these factors that affect the cooling rate:

Preheating application and temperature

What is Preheat in Welding?

Setting and maintaining a minimum preheating temperature helps to slow down the cooling rate to prevent or minimize the formation of brittle microstructures. To ensure effective preheating, please use a sufficient temperature for the material. Materials with higher alloy content-and therefore higher hardenability-require higher preheating temperatures. It is also important to set the correct preheat temperature throughout the thickness of the material, not just to reach the surface temperature. This can be done by induction heating or by heating the material and holding it at a temperature per inch of material thickness for half an hour (for example: a 4 inch (102 mm) board would benefit from being held at the preheat temperature for two hours). It is also important to heat at a sufficient distance from the weld. A good rule of thumb is 3 inches in all directions, although larger welds can benefit from greater spacing.

Welding Preheat Calculator

Interpass temperature

It is also important to set the highest interlayer temperature to avoid slow cooling and processing of quenched and tempered low-alloy steel. The welding “resets” the HAZ microstructure, which is carefully created by quenching and tempering. Therefore, it is ideal to minimize the heat-affected zone by adjusting the interlayer temperature to the limit recommended by the steel manufacturer. In addition, certain steels (such as ASTM A514) are prone to cracking when reheated, and excessive interlayer temperature will increase this risk.

Heat input

This is the energy applied to the welded structure per unit length during the welding process. In North America, heat input is usually expressed in kilojoules per inch, but it can also be expressed in joules per millimeter. The increased heat input slows down the cooling rate and produces a coarse-grained structure with lower tensile strength and toughness. The reduced heat input will speed up the cooling rate and produce a finer grain structure, which to a certain extent has higher tensile strength and toughness.

 Too little heat input will impair ductility and toughness.

What is heat input in welding, its formula, online calculator, and unit

Suggestion for welding bead placement of welding sequence

 The stitching sequence is another factor that affects the results of welding low alloy steel. There is one HAZ for each welding pass. A wide weld bead with minimal thickness and penetration depth promotes additional grain refinement and forms a finer microstructure throughout the weld cross-section-and-in the previously deposited layer. It is recommended to use more thinner and wider channels than larger and thicker channels.  tempered bead can also be used for extremely high-strength alloys. The technique involves placing the weld bead on top of the finished weld to refine the HAZ and cover the microstructure of the weld and provide good toughness, and then to grind these welds away, leaving the previous layer of refined weld metal underneath.

Final Post weld heat treatment

 Due to the higher hardness of the material, low alloy steels often require post heat treatment (PWHT), which leads to higher welding stress.

 These stresses can be eliminated by heat treatment after welding, but the reaction of the filler metal to the heat treatment is different from that of the base metal. It is important to select filler metals that can maintain sufficient tensile strength and toughness after post-weld heat treatment.

 Contact the filler metal manufacturer to determine the post-weld heat treatment capability of the filler metal, especially if the treatment is used for a long time.

 In some cases, it is recommended to maintain the preheat or minimum temperature between two passes after welding-approximately one hour per inch of substrate thickness -. Informally called “hydrogen bakeout”, this helps to speed up the diffusion of hydrogen in the weld metal before maintenance.

What is post weld heat treatment (PWHT) or stress relieving?

In short,

 low-alloy steel is easier to harden than unalloyed steel, which makes it important to control the hydrogen and cooling rate when welding these materials. Choosing the filler material corresponding to the characteristics of the base metal, the correct preheating and interlayer temperature, and the heat supply during the welding process contribute to success.

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