§1Joining by fusion
Welding melts the edges of the parts and lets them solidify together, usually with added filler of similar composition, so the join becomes continuous with the parent metal — one piece, not two joined.
This fusion is what sets welding apart from soldering and brazing, where the parts stay solid and a lower-melting filler bonds them. In welding, the parent metal itself melts into a common pool with the filler, so a good weld is as strong as the parent — the most permanent, load-bearing join available, and the backbone of steel structures, pressure vessels, ships and machinery. But melting the parent brings the difficulties this page covers: intense heat must be delivered just where wanted (§2–3), that heat alters the metal around the weld (§4), it makes the structure shrink and distort as it cools (§5), and whether a steel welds soundly at all depends on its chemistry (§6). Fusion gives welding its strength and its problems in equal measure.
Contents§2The arc processes
Most welding melts the metal with an electric arc struck between an electrode and the work — a family of processes differing in how the electrode and the shielding are provided.
An arc is an intensely hot electrical discharge, easily hot enough to melt steel, and the main processes harness it differently. Manual metal arc (MMA, “stick”) uses a consumable flux-coated rod that both melts as filler and releases a shielding gas and slag — simple, portable, all-position. MIG/MAG (gas metal arc) feeds a continuous bare wire and shields the pool with a separate gas — fast and easily automated, the production workhorse. TIG (gas tungsten arc) strikes the arc from a non-consumable tungsten electrode under an inert gas, with filler added by hand — slow but clean and precise, for thin work, aluminium and quality joints. Gas (oxy-acetylene) welding uses a flame instead of an arc, now mostly for repair and brazing. What every one must do is shield the molten pool from air — which would embrittle it with oxide and nitride — whether by flux, slag or gas. The choice trades speed, portability and quality.
Contents§3Heat input
How much heat a weld puts into the metal per unit length — the heat input — governs the weld’s size, its cooling rate, the heat-affected zone and distortion. It is computed from the arc power and travel speed.
Welding at 25 V and 200 A, travelling 300 mm/min, the heat input is (25 × 200 × 60)/(300 × 1000) = 1.0 kJ/mm. This single figure captures how hot and how slow the weld is: a high heat input (more power, slower travel) makes a big weld pool that cools slowly, giving a softer, wider heat-affected zone and more distortion; a low heat input (less power, faster travel) cools fast, giving a narrower zone but risking a hard, brittle weld and lack of fusion. So heat input is the master control an engineer specifies — high enough to fuse properly, low enough to limit the damage — and it ties directly to the cooling rate that decides the weld’s and the heat-affected zone’s final structure (§4). It is the welding equivalent of the heat treatment the materials pages describe, applied locally and in seconds.
§4The heat-affected zone
Around the melted weld lies a band that did not melt but was heated enough for its structure to change — the heat-affected zone, often the weakest or most brittle part of a welded joint.
The weld pool reaches melting temperature, but the parent metal beside it is taken through a range of high temperatures short of melting — and, as the heat-treatment pages show, heating steel and then cooling it changes its microstructure. In this heat-affected zone (HAZ) the metal may be hardened (if it cools fast, forming brittle martensite), softened (if a previously hardened or work-hardened metal is annealed by the heat), or coarsened in grain — none of it what the designer chose. Because these changes happen without melting, they are invisible, yet the HAZ is frequently where a welded joint cracks or fails: a hard, brittle HAZ in a hardenable steel is a classic source of cold cracking. Controlling it is the point of managing heat input (§3) and preheat (§6): the aim is a HAZ neither too hard nor too soft. Every fusion weld has a HAZ; sound welding is largely about keeping it benign.
Contents§5Distortion and residual stress
A weld heats a small region intensely while the surrounding metal stays cool, and the uneven expansion and contraction leaves the part distorted and internally stressed.
The weld and its surroundings expand when heated and shrink when they cool, but because only a narrow strip is heated, that strip is restrained by the cold metal around it — so as the weld cools and tries to contract, it cannot freely, and the result is both distortion (the part bends, twists or pulls out of shape toward the weld) and residual stress (locked-in tension in and around the weld, balanced by compression further out). These can warp a fabrication out of tolerance and, with the HAZ, drive cracking. The remedies are practical: balance welds about the neutral axis or alternate sides so shrinkages cancel; clamp or pre-set the parts to pull against the expected movement; use lower heat input and fewer, smaller passes; weld in a planned sequence (back-step, skip) to spread the shrinkage; and stress-relieve afterward by heating if the residual stress matters. Distortion is the inevitable companion of local heating, managed by symmetry, restraint and sequence rather than eliminated.
Contents§6Weldability and carbon equivalent
Whether a steel welds soundly or cracks depends chiefly on its chemistry, summed up in the carbon equivalent — the same measure the materials pages used to judge hardenability.
The more a steel’s carbon and alloy content, the more its HAZ hardens into brittle martensite on the fast cooling a weld gives — and the more it is prone to cold cracking. The carbon equivalent (CE) rolls carbon and the alloying elements into one number, and the rule of thumb is that a CE below about 0.4 welds readily, while a higher CE needs care. A plain 1045 steel, CE ≈ 0.38, is near the limit and weldable with modest precautions; an alloy 4140, CE ≈ 0.77, is well above it and will crack in the HAZ unless it is preheated — warming the whole joint before welding so it cools more slowly, avoiding brittle martensite — and often post-weld heat-treated. So weldability is read straight off the composition: low-carbon structural steels weld freely, while hardenable alloy and high-carbon steels demand preheat and controlled cooling. The carbon equivalent tells the welder which situation is at hand before an arc is struck.
§7Quick reference
The working core of the page on one card rack.
Fusion
parts melt & fuse (not brazing)
weld as strong as parent
Arc processes
MMA stick · MIG/MAG · TIG
all must shield the pool
Heat input
HI = VI·60/(v·1000) kJ/mm
25 V 200 A @ 300 → 1.0 kJ/mm
HAZ + distortion
heat-altered band · warping
manage by heat & sequence
Weldability
CE < 0.4 welds freely
4140 CE 0.77 → preheat