BETTER UNDERSTANDING STEEL FOR ITS WELDING
Carbon steels are steels containing up to 2% carbon plus some other elements, most of which are present in small amounts, except for silicon (0,6%) and manganese (up to 1,6%) which are needed to de-oxidize the weld pool. These steels are also referred to as non-alloy steels. Carbon steel can be found in many applications since it is easy to shape, machine, weld and is very economical. In addition, its mechanical properties can easily be altered through various heat treatment processes (annealing, normalizing, hardening, etc.).
Alloy steels contain significant amounts of alloy elements other than carbon, plus frequently-allowed amounts of manganese, silicon, sulfur and phosphorous which are added to modify the alloy’s physical and mechanical properties. Low alloy steels are those in which the alloy elements do not exceed 5%, and high alloy steels are those in which the elements exceed over 5%. Alloy steels are used when higher mechanical properties are required or when a specific property is needed.
Classification
There are a number of steel classification systems, but the most frequently used and best known are those of the SAE (Society of Automotive Engineers) and AISI (American Iron and Steel Institute). Their classifications systems are based on a code that indicates the approximate composition of the steel. The SAE system is essentially the same as that of the AISI.
XXXX The first number indicates the primary class or group
XXXX The second number indicates the sub-class based on the main alloy elements
XXXX The third and fourth numbers indicate the percentage of carbon, in tenths of one per cent
The following table shows the various steel classifications according to the SAE or AISI.
10XX | Carbon steels | Carbon steels |
11XX | Carbon steels | Resulfurized carbon steels |
12XX | Carbon steels | Resulfurized and rephosphorized carbon steels |
13XX | Carbon steels | 1,75% manganese carbon steels |
15XX | Carbon steels | 1,0 – 1,65% |
23XX | Nickel steels | 3,5% nickel steels |
25XX | Nickel steels | 5% nickel steels |
31XX | Nickel – chromium steels | 1,25% nickel and 0,65% chromium steels |
32XX | Nickel – chromium steels | 1,75% nickel and 1,07% chromium steels |
33XX | Nickel – chromium steels | 3,5% nickel and 1,55% chromium steels |
34XX | Nickel – chromium steels | 3% nickel and 0,77% chromium steels |
40XX | Molybdenum steels | 0,20 – 0,25% molybdenum steels |
41XX | Molybdenum steels | 0,60 – 0,95% chromium and 0,12 – 0,30% molybdenum steels |
43XX | Molybdenum steels | 1,82% nickel ; 0,5 – 0,8% chromium and 0,25% molybdenum steels |
43BVXX | Molybdenum steels | 1,82% nickel ; 0,5 – 0,8% chromium ; 0,12 – 0,25% molybdenum |
43BVXX | Molybdenum steels | 0,03% min. vanadium and 0,0005 – 0,003% boron steels |
44XX | Molybdenum steels | 0,40 – 0,52% molybdenum steels |
46XX | Molybdenum steels | 0,85 – 1,82% nickel and 0,20 – 0,25% molybdenum steels |
47XX | Molybdenum steels | 1,05% nickel ; 0,45% chromium and 0,20 – 0,35% molybdenum steels |
48XX | Molybdenum steels | 3,5% nickel and 0,25% molybdenum steels |
50XX | Chromium steels (not stainless) | 0,27 – 0,65% chromium |
51XX | Chromium steels (not stainless) | 0,80 – 1,05% chromium steels |
52XX | Chromium steels (not stainless) | 1% min. carbon and 1,45% chromium steels |
61XX | Chromium – vanadium steels | 0,60 – 0,95% chromium and 0,10 – 0,15% min. vanadium steels |
81XX | Nickel – chromium – molybdenum | 0,4% chromium ; 0,30% nickel and 0,12% molybdenum steels |
86XX | Steels | 0,5% chromium ; 0,55% nickel and 0,20% molybdenum steels |
87XX | Steels | 0,5% chromium ; 0,55% nickel and 0,25% molybdenum steels |
88XX | Steels | 0,5% chromium ; 0,55% nickel and 0,35% molybdenum steels |
93XX | Steels | 1,2% chromium ; 3,25% nickel and 0,12 |
94XX | Steels | 0,4% chromium ; 0,45% nickel and 0,12% molybdenum steels |
97XX | Steels | 0,2% chromium ; 0,55% nickel and 0,20% molybdenum steels |
98XX | Steels | 0,8% chromium ; 1,00% nickel and 0,25% molybdenum steels |
92XX | Silicon – manganese steels | 1,4 – 2,0% silicon and 0,65 – 0,85% manganese steels |
XXBXX | ”B” denotes the addition of boron | |
XXLXX | ”L” denotes the addition of lead |
4340 43XX = Steel containing 1,82% nickel ; 0,5 to 0,8% chromium and 0,25% molybdenum.
XX40 = 0,40% carbon content in the alloy.
Weldability of steels
Welding annealed steels
Annealing is a heat treatment for ferrous alloys that consist of heating the material for a given time and then cooling it down at a controlled rate to restore its mechanical properties that were altered during forming through rolling, stamping, etc.
When welding annealed steels, no modification of the base metal will occur beyond the heat affected zone (HAZ). Within the HAZ, progressive structural modification will occur, depending on the temperature reached. Such changes can result in hardening of the HAZ, depending on the carbon and alloy element content of the welded metal, and therefore a risk of cold-cracking.
It is better to make several small passes or beads, rather than one wide pass to reduce the size of the HAZ and thus obtain better mechanical properties for non hardenable steels. However, for hardenable steel the alloy must be preheated in accordance with the Preheating section to minimize the risk of hardening.
Welding of hardened or hardened and tempered steels
Hardened steels have a martensitic structure which gives them a very low toughness strength but high hardness and mechanical strength. The higher the carbon content, the harder and more brittle the martensite, thus the greater the risk of cold-cracking. When the carbon content is less than 0.18%, the risk of martensite cracking is negligible.
Martensite becomes brittle in the presence of hydrogen, even if the carbon content is less than 0.18%, because the hydrogen trapped within the metal deposited during cooling produces weld stresses. To remedy the problem, it is recommended that pieces be throughly cleaned to remove all traces of hydrocarbons (oils, greases, soaps, etc.), and that basic type filler products be used (Sodel 32, Sodel 127, Sodel 318) to minimize the presence of hydrogen, or that specialized filler products (Sodel 333, Sodel 335) to be used for steels that are hard to weld.
Another technique to prevent cold-cracking during welding of hardened steels is to preheat the part. Preheating a low hardening steel when welding slows down cooling to prevent the part. Preheating a low hardening steel when welding slows down cooling to prevent hardening and to help the hydrogen to escape. If the steel hardens when welded, hardening cannot be prevented by preheating, but it will allow hydrogen to escape from the weld. For maximum efficiency, it is better to preheat the part in accordance with the preheating section, weld it with high current within the range of the electrode, and cover the part with an insulating material after welding. In addition, when welding hardened steels, hardness and tensile strength drops off progressively in part of the HAZ, depending on the temperature.
Preheating
The preheat temperature varies according to the type of steel and filler product. The following recommendations can be used to determine the preheat temperature :
-Find the recommended preheat temperature from the steel 1 table according to the chemical composition, carbon equivalent, and carbon content.

These temperatures are recommended for 4 inch (100 mm) steels for groove welds or 2 inch (50 mm) steels for fillet welds. If the thickness varies, the temperature should be readjusted by increasing or decreasing it by 110°F / inch (2.4°C / mm) as the thickness increase or decreases.
-The preheat temperature should not exceed 900°F (480°C).
-For thin sections, care must be taken to avoid preheat distorsion.
-With steels having a carbon equivalent over 0.70, it is better not to lower the recommended preheat temperature, even for thin sections, because of the high temperability of such steels.
-When using rutile filler product (Sodel 31, Sodel 314), the calculated required temperature should be increased by 300°F (150°C).
-For sections thicker than 1 inch (25 mm), use only basic filler products (Sodel 32, Sodel 127, Sodel 318) or specialized filler products for hard-to-weld steels (Sodel 333, Sodel 335) to minimize cold-cracking risks.
-Preheat until the required temperature has been reached on each side of the joint at a distance greater than or equal to the thickness of the section to be joined, but no less than 3 inches (75 mm). Maintain this temperature throughout the welding operation.
-To increase the chances of success after welding, cover the part with an insulating material such as mineral wool, and allow it to cool gently.
-No preheating is needed for steels with a carbon equivalent of less than 0.45 and a thickness of 1/2 inch (13 mm) or less.
-When using specialized filler products (Sodel 333, Sodel 335) for hard-to-weld steels, much lower preheat temperatures can be used, and welding can even be done cold in certain cases as indicated in the steel 2 table.

Steel 2 table : Preheating for specialized electrodes
Plate thickness | Plate thickness | Plate thickness | Plate thickness | |
Carbon Equivalent | 0 – 1/2” (0 – 13 mm) | 1/2 – 1” (14 – 25 mm) | 1 – 1 1/2” (26 – 38 mm) | 1 1/2 – 2” (38 – 50 mm) |
0.46 – 0.55 | none | none | none | none |
0.56 – 0.60 | none | none | 200°F (95°C) | 300°F (150°C) |
0.61 – 0.72 | 200°F (95°C) | 200°F (95°C) | 350°F (175°C) | 450°F (230°C) |
0.72 and over | 200°F (95°C) | 350°F (175°C) | 450°F (230°C) | 500°F (250°C) |
It is recommended that the plate thickness be considered double for fillet welds.
Principal weld defects
Cold-cracking
A cold crack is a crack that develops when the weld is cold. It is also called deferred cracking or underbead cracking. It may develop up to 48 hours after welding. It generally occurs beneath the bead and, rarely, in the deposited metal.
Three factors must be present at the same time for cold-cracking to occur :
-hydrogen must be trapped beneath the weld bead;
-the structure must be brittle;
-stresses must be present.
Hydrogen can come from moisture on the piece, from the filler metal, from the coating, from the flux (in submerged arc welding) or from the shielding gas. Hydrocarbons (oils, greases, soaps, etc.) on the piece or filler metal are another source of hydrogen.
The problem is that hydrogen is soluble in liquid steel but not at low temperatures. Since the weld cools down from the surface inward, it drives the hydrogen below the surface where the temperature is still high enough to retain the hydrogen. During cooling, the hydrogen that is no longer soluble becomes trapped in the bead and causes increased stresses within the weld.
The presence of a brittle structure like martensite is highly undesirable in terms of cold-cracking because it cannot absorb stresses, instead it cracks. The higher the carbon equivalent, the greater the likelyhood of having a martensitic structure. The cooldown rate also affects martensite formation, and the carbon content determines the hardness as well as the ductility of the martensite.
Stresses result from contraction of the weld pool and the HAZ during cooldown. Restrained parts also induces additional stresses as do martensitic transformation around the bead and the weld sequence.
Hot-cracking
Hot-cracking occurs when the weld cools. It always appears in the middle of the bead. It is caused by one or more of the following :
-Chemical elements : sulfur / copper / phosphorus
-Other : highly restrained area / bead too narrow / poor fit-up (filled weld) / too high welding speed / weld pool too deep and not wide enough
Porosity
Gas may form in the weld pool for many reasons. The gas is present in the form of round, smooth holes called porosities. They occur when the metal solidifies too quickly to allow time for the gas to escape.
A gas inclusion can be caused by :
-chemical reactions during welding;
-plates or electrodes that are too high in sulfur content;
-excessive moisture in the electrode coating or on the plates;
-an arc that is too long;
-improper welding current or polarity
Good welding methods and filler products with a composition appropriate to the base metal will prevent porosity formation. Preheating helps to de-gasify the weld by slowing the solidification rate, and a good cleaning before and during welding will help to produce porosity-free welds.
PRACTICALTIPS FOR WELDING STEEL
1-Select the appropriate filler metal: mechanical properties, corrosion resistance, compatibility between filler metal and base metal, etc.
2-Use basic type filler products (Sodel 32, Sodel 127, Sodel 318) to minimize the presence of hydrogen.
3-Preheat the part if necessary, taking into consideration the alloy elements, carbon percentage and thickness.
4-Minimize the effects restraint by using an appropriate welding sequence.
5-Make narrow passes where possible instead of wide passes
6-Try to minimize the width of the heat affected zone.
7-Use a filler product containing manganese when welding resulfurized steels.
8-For optimal results with the SMAW process, it is important to allow time for the shielding gas to set up well before starting to move.
9-The factors that cause hardening of steel are : alloy elements / alloy carbon content / overheating in the HAZ (heat affected zone) / cooling too quickly / low initial temperature in base metal.
10-To eliminate porosities, inclusions, accelerated corrosion and risk of cracking, always thoroughly clean the parts to be joined before and during welding to remove all traces of impurities.