How to Use a Welding Chipping Hammer

A welding chipping hammer breaks the hard glassy crust of slag off the top of a finished weld. You wait until the bead stops glowing, hold the hammer near the end of the handle, and strike along the weld at a shallow angle with the chisel end, so the crust peels off sideways instead of firing back at your face. The pointed end lifts slag out of corners, fillet toes and undercut where the chisel rides straight over. Then a wire brush takes the film that is left.

What a chipping hammer actually is

It is a small, light hammer - usually about 10 to 11 inches long and well under a pound - with two working ends set at right angles to each other. One end is a flat chisel, ground like a wide flathead screwdriver. The other is a cone or a vertical pick. Neither end is meant to hit hard. Slag is brittle, and the hammer's job is to shock it loose, not to drive it.

The handle on most factory hammers is a coiled steel spring, and that is a design decision rather than decoration. The coil flexes on impact and takes the sting out of a shock that would otherwise go into your wrist a few hundred times a shift. Wooden and solid steel handles exist and work; they are simply harsher. What no standard chipping hammer has is an insulated handle, so it is not a tool to rest against a live electrode holder.

Which welding processes leave slag, and which do not

Slag comes from flux. No flux, no slag, no reason to pick up the hammer. TWI list slag inclusions as a defect associated with MMA, flux-cored and submerged arc welding, and add that they can also occur in MIG welding - worth knowing, because plenty of guides state flatly that MIG never leaves anything behind.

Those silicon islands on a MIG weld deserve a sentence of their own. They are small brown glassy spots thrown out by the deoxidisers in the wire, not a crust, and a wire brush removes them. Take them off if you are painting or laying another pass over the top, because they behave like any other inclusion. Leave them if the weld stays bare and nothing else is going on it.

How to use a welding chipping hammer, step by step

1. Let the weld stop glowing. Slag releases better once it has contracted, and on a well run stick bead a good deal of it lifts or pings off by itself as the weld cools. Chipping at a bead that is still red also throws hotter, stickier debris. Ten or twenty seconds is usually enough on thin material.
2. Secure the work before you swing. A part that skates across the bench takes your free hand with it. Clamp it - this is exactly what a welding clamp is for - and keep your holding hand behind the line of the strike, never in front of it.
3. Work along the bead, not across it. Lay the chisel end almost flat, somewhere around 15 to 30 degrees to the plate, and drive it under the edge of the crust so the slag peels along the weld. Striking straight down at 90 degrees shatters it instead and sends fragments in every direction, including back at your face.
4. Aim the strike away from yourself and away from everyone else. Slag leaves in the direction the chisel pushes it. Point it down the bench or into a corner. If someone is working opposite you, a piece of slag travelling at chest height is your responsibility, not theirs.
5. Use the cone end for what the chisel cannot reach. Fillet toes, inside corners, undercut and the crater at the end of a run all hold slag that a flat chisel skips over. Pick it out rather than hammering at it.
6. Finish with a wire brush, then look at the weld. The hammer takes the crust, the brush takes the film. A clean bead is also the only bead you can inspect - porosity, undercut and lack of fusion all hide underneath slag.

Why the slag has to come off before the next pass

On a single pass weld, chipping is housekeeping. On a multi-pass weld it is part of the procedure. TWI are blunt about it: it is crucial to remove all slag before depositing the next run, and that can be done between runs by grinding, light chipping or wire brushing. Slag left in the joint gets buried by the following bead and becomes a slag inclusion - a void with a hard non-metallic lump sitting in it, which shows up on radiography and fails the weld.

Two places catch people. The first is the toe of the previous run, where the bead meets the plate at a sharp angle - if that pass was laid with too little overlap or left undercut, slag sits in the notch and the chisel skids over the top of it. That is cone end work. The second is the crater at the end of a run, where the slag is thickest and most stubborn. TWI also note that with difficult electrodes or in narrow vee joints it can take a grinder between layers rather than a hammer, so if the slag will not come out, stop hitting it harder and change tools. Most of this is really a stick welding problem, and the rod you chose decides how hard the slag fights you.

Five mistakes worth not making

• Chipping with the helmet down. A welding filter is dark and it is not an impact shield. Z49.1's own explanatory note says welding helmets will not protect against the severe impact of fragmenting grinding wheels or abrasive discs. Lift the hood, keep the spectacles on.
• Hitting it like a nail. More force does not remove more slag. It shatters the crust, drives fragments into the weld toe and marks the plate. A light, quick strike at a shallow angle does more with less.
• Chipping toward the edge of the bench with people behind it. Slag travels further than most people expect, and it is still hot when it gets there.
• Leaving the cone end dull. A rounded point cannot get under anything, so it bounces and you swing harder to compensate. Two minutes on a grinder fixes it.
• Reaching for it after MIG or TIG. There is no slag to remove. All you can do is drag a hardened chisel across a finished weld and score it. Use a brush.

Chipping hammer, wire wheel or needle scaler

For one bead at a time the hammer wins on speed and control, which is why it lives in a welder's pocket rather than on a shelf. Over a long fabrication a needle scaler does in a minute what the hammer does in ten. Neither of them replaces the brush.

Keeping it working

• Keep the cone pointed and the chisel square. A few passes on a bench grinder, dipping often. If the tip turns blue you have drawn the temper out of the last few millimetres and it will round over again quickly.
• Check where the head meets the handle. Coiled spring handles work loose. A head that leaves mid-swing is a projectile.
• Do not use it as a punch or a pry bar. The chisel end is thin and hard, and hard thin steel chips.
• Wipe it and oil it. Slag dust holds moisture and the tool lives in a damp corner of most shops. A rag and any light oil is enough.
• Keep it clear of the electrode holder. Nothing on a standard chipping hammer is insulated.

Building your own, and what the usual rebar plan gets wrong

Making a chipping hammer is a reasonable first fabrication project, and the standard internet version - two lengths of rebar, cut, dressed and welded at 90 degrees - does work. Three of the claims that travel with it do not.

• Rebar is not stainless steel. The ordinary grade is ASTM A615, whose full title is Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement. It is plain carbon steel, it rusts, and A615 sets no upper limit on carbon content at all.
• Which is exactly why welding it is not casual. The grade written for welding is ASTM A706, which in its own words limits chemical composition and carbon equivalent to enhance the weldability of the material, and points the reader at AWS D1.4/D1.4M, the structural welding code for steel reinforcing bars. Unknown-carbon A615 welded cold can crack at the joint, and on a hammer that means the head parting company with the handle.
• Quenching hot rebar in water does harden it - that is the problem, not the benefit. Some guides call this unproven. It is the oldest heat treatment there is. Quenching a carbon steel bar makes it hard and brittle at the same time, and a brittle chisel tip spalls instead of wearing. If you harden the tip, temper it afterwards.

If you want to build one anyway, the honest recipe is short. Take a piece of 12 mm bar around 5 to 6 inches long for the head. Cut a flat chisel on one end and four steep facets on the other to form a cone, then dress both on a grinder, dipping often so you do not blue the tips. Weld the head at 90 degrees to a handle - preheat if the bar is unknown carbon, tack it, check the angle, then weld both sides of the joint and inspect the weld before you trust it. An old hammer handle or a length of coil spring both work. Finish with oil rather than paint, because paint burns off the first time you set it down on hot work.

It is a good beginner project because it is small, forgiving and useful when it is done. If you want more in that vein, there is a longer list of welding project ideas sorted by skill level.

Frequently asked questions

Sources

• ANSI/AWS Z49.1:2021, Safety in Welding, Cutting, and Allied Processes (free PDF) - clause 4.2.1.1 and the explanatory note E4.2.1.1
• OSHA 29 CFR 1910.133 - eye and face protection, including the side protection requirement
• ANSI/ISEA Z87.1 - the current edition and what the Z87 and Z87+ markings mean
• TWI - Defects / Imperfections in Welds: Slag Inclusions
• ASTM A706/A706M-22 - the reinforcing bar grade specified for welding, and its carbon equivalent limit
• TimWelds - Stick Welding Basics: Full Tutorial