AWS D3.6M: The Underwater Welding Code and Its Four Classes
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Welding & Cutting | Cathodic Protection & Anodes I was standing at the edge of a pump intake in 2009, watching a diver burn a pad-eye onto a trash rack with a 7018 rod, when an engineer over the phone asked what class we were running. He didn't know the code existed. Most people don't. AWS D3.6M is the American Welding Society's code for underwater welding. It covers both wet welding and dry hyperbaric welding, and it sorts every underwater weld into one of four classes: A, B, C, and O. That classification isn't a rating of difficulty. It's a set of acceptance criteria, testing requirements, and quality expectations that the engineer or owner picks based on how the weld will be loaded and what it's holding. I've spent 28 years doing this work, on dams, bridges, water tanks, and power plant intakes. The code didn't exist when I started; we welded to whatever the contract said, which was often nothing. Now, if you're working for a knowledgeable owner, the contract will say 'per AWS D3.6M Class B' or 'Class A wet welding.' If it doesn't, you should ask yourself why.
What AWS D3.6M Actually Covers
First, it's not a federal regulation. OSHA regulates commercial diving under 29 CFR 1910 Subpart T, but that section talks about the diver's safety and the diving operation, not weld quality. AWS D3.6M is a consensus standard written by engineers, divers, and fabricators. It becomes law when a project specification references it. Then it's as binding as any code. The code applies to welding performed underwater, whether the welder is fully immersed or working inside a one-atmosphere habitat. It also covers the welder's qualifications, the procedures, and the nondestructive or destructive tests used to verify the weld. The definitions are finicky, but the core message is simple: you have to prove the weld will do its job before it's trusted.
The Four Classes: A, B, C, and O
Class A is the top tier. The code writers designed it to produce welds with mechanical properties and soundness comparable to above-water welding per AWS D1.1. That means full penetration if the joint calls for it, no cracks, no unacceptable porosity, and a minimum level of ductility and tensile strength. Class A welds are typically specified for critical structural members, like main load path repairs on a bridge or a penstock. To achieve Class A, you often need a dry environment or a controlled hyperbaric chamber. It's expensive, slow, and unforgiving. But if the load is dynamic and the part is critical, that's what you want. Class B is the workhorse. It has less stringent acceptance criteria than Class A, but it is still a real structural weld. You can use it for many underwater repairs where the load is known and the consequences of a flawed weld are manageable. A lot of wet welding with flux-cored wire comes out at Class B, provided the procedure is qualified. That's the class I see most often in the field, especially on dams and pile splices. Class C is for welds that aren't carrying any meaningful load. Think of an attachment for a temporary cable, a vent line, or a datum plate you're installing for future inspections. The code sets no specific acceptance criteria for Class C, but that doesn't mean you weld sloppy. It means the engineer is allowed to define what's good enough, or in many cases, just trust the diver's skill. Class O is the catch-all. 'O' stands for 'other.' It's for applications that don't fit A, B, or C, or where the specifier wants to impose test requirements that fall outside the usual classifications. You'll see Class O used when the owner has their own acceptance criteria based on the particular service, maybe a crack-stopping detail or a special corrosion allowance.
Picking the Class, and Living With It
So who decides? Usually a structural engineer, with input from the owner's corrosion and maintenance people. It's not the diving supervisor's call, but you'd better know what class you've committed to before you dive in. Class A on a wet weld is a setup for failure unless you're using a habitat. Class B on a critical fatigue detail could get you sued. Here's the practical reality: Class A often requires a dry hyperbaric chamber or a damn good cofferdam. That changes your whole diving spread. You're no longer just a diver with a stinger; you're a hyperbaric weld shop operating in a goldfish bowl. Class B wet welding, on the other hand, can be done with a standard dive rig, an electrode oven, and a welder who knows his technique. The cost difference is enormous. I've seen jobs where specifying Class A doubled the budget and added three weeks to the schedule, for a weld that would've been fine as Class B. But here's the flip side. I've also seen owners cheap out and call something Class C just to get a notch repaired, and then a year later that notch is a crack running up the chine. The classification isn't a hack. It has to match the service. When in doubt, get the engineer to write the requirement down, with a signature.
Testing, Inspection, and Qualifying
Compliance with D3.6M isn't just making the weld pretty. Under the code, you have to qualify the welding procedure (WPS) and the welder, before production starts. That means making test coupons underwater, in the same position and environment you'll be working in, then cutting them perpendicular to the weld and breaking them, bending them, or taking tensile samples. For Class A, you're also looking at radiography or ultrasonic testing on a large percentage of the production welds. Class B typically requires visual inspection and maybe some random NDT, depending on what the contract says. A lot of divers don't realize that their commercial dive certification, even one from ADCI, doesn't make them an underwater welder. You have to have a separate welder qualification to D3.6M, or to the applicable AWS code for the welding process. I've worked with guys who could lay a beautiful bead but couldn't pass the bend test because they had inclusion. And I've worked with the opposite: a great test coupon and a lousy job in zero visibility. The code only tells you how to test. It doesn't tell you how to judge a welder's nerve or judgment in the water. That comes with time. A note to inspectors reading this: don't just stand on the deck asking for a copy of 'the dive plan.' Get the WPS, get the welder's qualification record, and get the NDT results. If those aren't on the barge, the weld doesn't exist. It's that simple.
Frequently Asked Questions
Is AWS D3.6M a government regulation? No. It's a consensus standard published by the American Welding Society. It becomes legally binding when a project specification or contract references it. OSHA's 29 CFR 1910 Subpart T covers diver safety, not weld quality. What's the difference between Class A and Class B welds? Class A welds must meet mechanical properties and soundness comparable to AWS D1.1 above-water welding, and they require extensive testing. Class B has less stringent acceptance criteria but is still considered a structural weld for many underwater repairs. Can a Class C weld be used on a structural member? Class C is intended for non-structural attachments or temporary items, and the code sets no specific acceptance criteria. If someone calls a load-bearing repair Class C, that's a red flag. The engineer has to define the criteria explicitly. Do I need a commercial diving certification to do underwater welding? Yes, you need commercial dive training and certification, typically from a recognized program, plus a specific welder qualification to AWS D3.6M for the process and positions you'll be using. Recreational dive cards don't count.