T-1, or ASTM A514 is a high strength steel which is quenched and tempered to provide yield strengths of over 100,000psi (over 690MPa). The name “T-1” is a trademark of Arcelor Mittal and not an ASTM, AISI or part of other organization’s standard numbering system. ASTM A514 is primarily used as a structural steel and also used for pressure vessels designated as ASTM A517. The Procedure Handbook for Arc Welding groups A514 and A517 under the same category (Category D) and recommends the exact same processes and procedures.
Welding quenched and tempered steels such as T-1 (ASTM A514) can be challenging due its high strength and hardenability. If proper procedures are not followed you can end up with high hardness in the heat affected zone (HAZ). This can lead to premature or even immediate failure.
T1 steels (ASTM A514) have good weldability when proper procedures are followed. The four key elements in successfully welding T1 steel are:
- Selection of the correct electrode or electrode flux combination.
- Adequate welding procedure
- Using the recommended fabrication practices
- Using caution if/when applying postweld heat treatment
- Electrode and/or electrode/flux combination
When there is a need to match the strength of the T1 base metal, electrodes with Mn-Ni-Cr-Mo that can provide the same or similar strength and toughness are necessary. This would require the weld metal to have a minimum tensile strength of 100,000 psi and charpy v-notch toughness of at least 20 ft-lbs at -40ºF.
For submerged arc welding this would require the use of filler metals having an AWS classification of F11A6-ECM2-M2-H8. An example of this wire/flux combination is 880/LAC-M2 from Lincoln Electric. Other manufacturers also have wires and fluxes that provide the requirements of this classification.
The recommended solid wire (GMAW) for this type of material is ER100S-G/ER110S-G dual classified wire, which is capable of producing welds with 100,000 psi tensile strength.
Other wire/flux combinations and mig wires that yield lower strength and toughness levels than the base metal can be used depending on the design stress and application.
The use of under matching filler metals is permitted and in most cases recommended, but as stated above this is a design consideration that has needs to be approved by the engineering department in charge. Note that if you use a 70,000 psi minimum tensile electrode, such as an ER70S-6, the dilution from penetration will yield much better than a 70,000 psi tensile strength weld.
Welding of T-1 steels can also be done with other processes such as SMAW, FCAW and GTAW. Pay attention to design requirements and use the right classification of filler metal. Always use filler metals that can provide a low hydrogen deposit.
- Adequate welding procedure
The adequate preheat is usually recommended by the manufacturer of the steel. Depending on the grade this may vary by 25-50 ºF. Going by plate thickness, typical preheat temperatures would be as follows:
Up to ½” 50-100F
Over ½ to 1” 50-150F
Over 1 to 2” 150F-200F
Over 2” 200-250F
Please note that highly restrained joints may require higher preheat. Also, preheat should NOT exceed 400F for thickness up to 1-1/2” or 450F for thickness over 1-1/2”. Because this is a Q&T steel we also have to abide by maximum preheat and interpass temperatures and not just minimums.
When preheating, it is important that the temperature be taken 3 inches away from the joint, in every direction (all along the joint). If the plate thickness were to exceed 3 inches then the distance from the joint at which the temperature reading is taken would be the same or more as the thickness of the plate.
The welding procedure (amps, volts, travel speed, etc.) should be such to provide side wall and root fusion. Depth of penetration becomes an important variable when using under matching filler metals as the amount of dilution will dictated the strength of the weld. The higher the heat input from welding the slower the cooling rate (which is what we want). However, care must be taken when welding different thicknesses. If we are welding 2” thick T1 to ¾” thick T1, and we preheat to 300F, the heat input produced by welding is not a concern in the thicker section. However, for the ¾” thick section our heat input should not exceed 82 KJ/in. There are published tables by the manufacturers of A514 steels that provide these limits.
- Use the recommended fabrication practices
The concern for hydrogen induced cracking is high when we are welding thick sections of low alloy steel. Large thicknesses produce a high level of restraint. Combined with a susceptible microstructure and a threshold level of hydrogen cold cracking can occur. To learn more about hydrogen induced cracking see Factors Influencing Hydrogen Induced Cracking and Preventing Hydrogen Induced Cracking. The good news is hydrogen-induced cracking will not occur unless all three of these are present. We can’t do anything about the inherent restraint from the thickness of the base material. We can’t do much with the microstructure of the material. But we can affect the level of hydrogen.
Common sources of hydrogen are:
- Moisture in the electrode, flux, shield gas or environment
- Decomposition of cellulosic-type electrode coatings
- Contaminants containing hydrogen (i.e. grease, oil, cutting fluids, water, etc.) on the surface of the material to be welded
The electrodes mentioned above all meet low hydrogen levels required. The solid wire (GMAW) process is best as there is little to almost no possibility of a high hydrogen deposit based on the wire alone. With SAW we need to pay special attention to the flux as it is very susceptible to moisture pick up. Proper storage and handling is essential. Plate must be free of contaminants. Correct preheat must be used as this helps drive off surface moisture. Slow cooling also allows hydrogen to diffuse out of the welds.
- Use care if/when applying postweld heat treatment
PWHT should only be done if the changes to the base metal and HAZ microstructure will provide benefits. PWHT usually involves temperature above 700F for this steel. Slow cooling is not considered PWHT and is necessary. Slow cooling can be accomplished by the use of thermal blankets. However, the cooling rate achieved with proper preheat is sufficient as long as welding is done indoors in a controlled environment.
Consulting AWS D1.1 Structural Welding Code – Steel, we find the same information. If you are in need of qualified welding procedure specification (WPS) you will find out that these will need to be qualified by testing.
A514 is not a prequalified base metal to be used with prequalified welding procedures. However, D1.1 recognizes this steel as one to be used in structural components in Clause 4 – Qualification.
Per Table 4.9, ASTM A514 with thickness of 2-1/2 inches or under, the recommended electrode classification for SAW is F11XX-EXX-XX. Based on this requirement the recommendation made above would apply.
Also, Table 4.9 provides minimum preheat and interpass temperatures.
Up to ¾ inches 50˚F
Over ¾ thru 1-1/2 inches 125˚F
Over 1-1/2 thru 2-1/2 inches 175˚F
Over 2-1/2 inches 225˚F
Table 4.9 states in the footnotes that “For ASTM A514, the maximum preheat and interpass temperature shall not exceed 400˚F for thicknesses up to 1-1/2 inches inclusive, and 450˚F for greater thicknesses.
One final comment: Welding T-1 (ASTM A514) should not be complicated. Follow the recommendations above and avoid problems.