Weld joint types: butt, corner, tee and lap joints and their letter designations

Four joint types

The letter shows how the parts lie relative to each other. It is the coarsest classification, but any choice of weld starts from it. The letters are Cyrillic and come from the Ukrainian and Russian words for butt, corner, tee and lap joints.

How to read a designation

The letter and number together are the code for a joint variant, for example С17. The number is neither a thickness nor a length. It points to one variant that already contains the edge preparation form, the character of the weld (one-sided or two-sided) and the permitted thickness range. That is why С17 and С25 differ although both are butt joints: one is a single-sided weld with a bevel, the other two-sided with symmetric bevels.

Within one variant the dimensions change with the plate thickness: the root gap, the root face, the weld width, the height of reinforcement. They are taken from the dimension table for that designation; they are not part of the drawing mark, and neither the welder nor the designer invents them.

The graphic symbol of the weld on the drawing (the sign, leader line, leg, length and pitch) follows the drawing conventions, see KM and KMD drawing rules.

Edge preparation

Thickness decides whether it is enough to bring the ends together or metal has to be removed along the edge. Thin metal is welded with almost no preparation or with flanged edges. The thicker the plate, the larger the bevel needed, otherwise the weld will not fuse the whole thickness.

The designations use these forms. Flanged edges: the edge is bent up, and thin parts are welded without filler or with very little. Square edges: the parts are only set apart by the gap needed. Single bevel: a chamfer is cut on one part, the other stays square. Double-edge bevel: a V preparation. Two symmetric bevels on each edge: an X preparation for large thicknesses when the weld is made from both sides. Curved and broken bevels: the preparation shape is changed to cut the amount of deposited metal at very large thicknesses.

The general rule is simple: the more complex the preparation, the less filler metal it uses and the less the part distorts, but the more accurate the cutting and the more operations it needs. For small thicknesses this is not justified.

One-sided and two-sided welds, backings, locks

A one-sided weld is made from one side; a two-sided one from both. A two-sided full penetration weld requires the root of the first weld to be cleaned to sound metal before welding from the other side. For asymmetric joints, if the root of the first weld is gouged, the back-run weld may be enlarged to the size of the first.

Where the other side cannot be reached (a pipe, a closed section, a blind box), a backing is used. A removable backing is taken off after welding. A permanent one stays in the joint as part of it. In a lock joint, the backing is a specially prepared lip on one of the parts. The designation spells out this character in words: «one-sided on removable backing», «on permanent backing», «lock».

Thickness ranges: the main designations

The table lists only the designations for which the thickness range is not in doubt. It is not the full list of variants. The rest (for example several modifications of С8–С12, У4, У5, Т1, Т7, Т9) are explained in the text below; exact edge and gap dimensions for any designation are taken from the dimension table of the standard in force.

How to choose a type

The practical order is this. First fix the type by how the parts lie. Then eliminate unsuitable variants by thickness: thin plates up to 4 mm are welded flanged or square, from 3 to 60 mm come the variants with a bevel on one or both edges, from 8 mm upwards two-sided welds with two symmetric bevels become sensible. For the largest thicknesses (over 30 mm) curved and broken bevels are used.

Next look at access. If only one side is available, variants with a backing or lock are needed. If the piece can be turned over, a two-sided weld gives less distortion and less filler metal, but the root has to be back-gouged. Finally weigh the quality demand: loaded splices that need full penetration are made with butt welds and inspected; where full penetration is not needed, fillet welds are often enough.

For thin metal in corner joints with square edges (У4 and У5) the system has two variants each: one for small thicknesses (up to 6 mm for У4, up to 8 mm for У5) and a second one reaching 30 mm. For tee joints with square edges (Т1 and Т3) the ranges rise to a few tens of millimetres, but they differ between tables, so the exact limit is taken from the dimension table for that designation.

Fillet weld leg

For fillet welds in tee and lap joints the main size is the leg K. Its value and tolerance are set by the designer, not by the shop. The upper limit: no more than 3 mm for parts up to 3 mm thick inclusive, and for thicker parts no more than 1.2 times the thickness of the thinner part.

The lower limit depends on the thickness of the thicker member and the yield strength of the steel. For steels with a yield strength up to 400 MPa the minimum leg grows from 3 mm for thicknesses of 3–4 mm to 10 mm for 40–80 mm; for steels above 400 MPa up to 450 MPa it is 1–2 mm larger. The minimum leg does not exceed 1.2 times the thickness of the thinner part. These are guide values; exact values for a project are taken from the standards in force.

Leg tolerances, as a guide, depend on the nominal size.

Different thicknesses and edge misalignment

If the thicknesses of the parts at a splice differ, but by no more than the permitted difference, the weld is made as for equal thicknesses and the edge and weld dimensions are chosen for the greater thickness. If the difference is larger, the thicker part is bevelled on one or both sides down to the thickness of the thin one, and the dimensions are then chosen for the smaller. A sloping weld surface is permitted for a smooth transition.

Edge misalignment and additional conditions

Before welding, the permitted misalignment of the edges depends on thickness: up to 0.5 mm for parts up to 4 mm; up to 1.0 mm for 4–10 mm; 0.1 s but not more than 3 mm for 10–100 mm; 0.01 s + 2 mm but not more than 4 mm for parts over 100 mm (s is the thickness).

A few conditions that are easy to miss. In butt, tee and corner joints thicker than 16 mm made in site conditions, the nominal gap b may be increased to 4 mm, and the weld width accordingly. When welding out of the flat position, the height of the reinforcement may be increased by 1.0 mm for parts up to 60 mm and by 2.0 mm for parts over 60 mm. Where welds overlap and where defects are repaired, weld dimensions may grow up to 30 % of nominal. Convexity and concavity of a fillet weld are permitted up to 30 % of the leg, and concavity must not reduce the leg set by the design.

Which welding process this system is for

The С, У, Т, Н designations and the dimensions given apply to manual arc welding with coated electrodes. They could also be carried over to welding in carbon dioxide with wire 0.8–1.4 mm in diameter. For automatic submerged-arc welding and other processes the edge dimensions, gaps and tolerances are different and come from the documents for those processes; the manual welding tables must not be transferred to them.

The system also does not cover welded joints of steel pipelines, which have their own rules. In practice in the shop the edges of pipes and shells are cut to a process card, and the joint type is chosen by thickness, diameter and access.

What to put on a KMD drawing

The shop does not choose the weld type in place of the designer. The drawing should have: the joint designation (letter and number) or a clearly drawn edge preparation; for fillet welds the leg and length, for intermittent ones the length and pitch; whether the weld is made in the shop or on site; and the inspection requirement if it is higher than usual (for example ultrasonic for butt welds).

Edge and gap dimensions are confirmed by the dimension table for the chosen designation and the welding procedure. We agree weld types and their inspection with the customer while the drawings are developed: KM and KMD design, steel fabrication. How welds are checked in the shop is in the article on quality control of steel structures.

Standards and validity

Edge preparation for manual arc welding, gas-shielded welding, gas welding, TIG and beam welding of steels is described by DSTU EN ISO 9692-1:2014 «Welding and allied processes. Recommendations for joint preparation. Part 1». The standards catalogue lists it as in force with a withdrawal date of 01.01.2027, so check its current state and any replacement before the project. For submerged-arc welding there is a separate part of the same series (DSTU EN ISO 9692-2:2014).

Welded joints in steel structures are calculated to DBN V.2.6-198:2014 «Steel structures. Design standards» (in force; edition with Amendment No. 1 of 2022), and the requirements for welding execution and inspection come from DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2. Technical requirements for steel structures».

The validity and current edition of each of these documents must be checked. The С, У, Т, Н designations and the numbers in this article come from Soviet practice and still stand on many drawings; we do not present them as the requirement of a current standard, and the numbers in the tables should be checked against the current document chosen for the project.