NDT on machine frame welds: what to specify, and what not to

“NDT on all welds” is a note that feels responsible and behaves like a blank cheque. On a machine frame carrying sixty metres of fillet weld it can add a five-figure sum and several weeks, and most of that spend buys information about joints whose failure would not matter.

The proportionate approach is to name the welds that carry the loads you care about, choose the right method for each, and inspect those thoroughly.

The four methods, and what each can see

Visual (VT). Not glamorous, and it catches more real defects than anything else on this list: undercut, overlap, incorrect profile, wrong size, missing welds, arc strikes, visible cracks. It should be specified on 100% of welds, performed by someone qualified, and documented. It costs almost nothing.

Magnetic particle (MT). Finds surface and very shallow subsurface defects in ferromagnetic materials — which carbon steel machine frames are. Fast, inexpensive, works well on fillet welds where the critical flaws are cracks at the toe or the root. For the great majority of machine frame welds, MT is the right supplementary method.

Dye penetrant (PT). Surface-breaking defects only, any non-porous material. Slower and messier than MT on steel, and confined to surface flaws. Its place on a machine frame is limited — use it where the material is not ferromagnetic, such as a stainless component in an otherwise carbon steel structure.

Ultrasonic (UT). Volumetric. Finds defects through the thickness: lack of fusion, lack of penetration, internal cracks, laminations. This is the method for full-penetration butt welds in thick section, which on a machine frame means the joints where a major member butts into another and carries the load through. It requires a skilled operator, and the quality of a UT result depends on that operator more than on the equipment.

Radiography (RT). Also volumetric, produces a permanent image, excellent at finding porosity and slag. Poor at finding planar defects oriented unfavourably to the beam — which includes lack of fusion, the defect type that matters most in thick structural welds. It needs radiation exclusion zones, which on a busy shop floor means night shifts. On machine frames, UT is usually the better volumetric choice; RT earns its place when you need an archived image or when the customer’s specification demands it.

Matching method to joint

A workable default for a machine frame:

Joint	Method
All welds	VT, 100%, to code acceptance criteria
Fillet welds on primary load path	MT on a named list
Full-penetration butt welds in thick section	UT, 100% of the named joints
Welds in non-ferromagnetic material	PT
Welds requiring an archived image	RT

The word doing the work is named. A critical weld table on the drawing — three to six joints, each with method, extent and acceptance class — gives the shop a priceable scope and gives you inspection where it matters. How that table sits alongside the weld symbols is covered in weld symbols on machine frame drawings.

Choosing which welds are critical

Three questions identify them:

1. Does this joint carry a primary load path? The weld between a column and its base plate does. A cable-tray bracket does not.
2. Would failure be expensive or dangerous? A crack that drops a gantry beam is a different category from one that loosens a cover panel.
3. Is the joint inaccessible after assembly? A weld that cannot be re-inspected or repaired once the machine is built deserves attention now, even if its load is modest.

Most machine frames end up with four to eight genuinely critical welds. Inspecting those properly costs a fraction of blanket NDT and delivers far more useful information, because the inspector’s attention is concentrated where it matters instead of spread across brackets.

Acceptance criteria: cite, do not invent

Do not write your own acceptance limits. Cite the class from the code you have already invoked — AWS D1.1 has acceptance criteria for each method, and the ISO 5817 quality levels (B, C, D) serve the same purpose in the European system, with B the most demanding.

For machine frames, ISO 5817 level C on general welds and level B on the critical list is a sensible and defensible pairing. Which standard framework to invoke overall is covered in which welding standard to put on a machine frame drawing.

Citing a class means the argument about whether a particular indication is acceptable has already been settled by a committee, in writing, before your part existed. That is worth a great deal when a report comes back with a marginal indication at eleven o’clock at night in a different time zone.

Who does the inspection

This is where specifications tend to be vague and where the value sits.

Shop NDT is performed by the fabricator’s own qualified personnel. Cheaper and faster. Entirely legitimate, and it carries an obvious conflict of interest on a marginal call.

Third-party NDT is performed by an independent agency. Costs more, takes longer to schedule, and the report is evidence rather than an assertion.

The proportionate arrangement for a machine frame: shop VT on everything, third-party on the critical weld list. Write it that way explicitly, because “NDT to be performed” without naming who performs it will default to the cheaper option.

Specify the personnel qualification too — ISO 9712 or ASNT Level II as a minimum for the operator, Level III for procedure approval. A UT result from an unqualified operator is not a result; the method is more operator-dependent than any other on the list, as the ASNT qualification framework exists to address.

Timing, which is usually got wrong

NDT belongs after welding and before stress relief and machining. That ordering matters for a practical reason: finding a defect after the part has been through a furnace and spent forty hours on a boring mill is dramatically more expensive than finding it the week it was welded.

Some specifications also require re-inspection after stress relief on the most critical joints, on the basis that the thermal cycle can reveal or extend an existing flaw. On a high-value frame that is reasonable. On a routine one it is usually over-buying. The stress relief sequence itself is covered in stress relieving welded machine bases.

What to write

Non-destructive testing. Visual inspection to AWS D1.1 on 100% of welds, by personnel qualified to ISO 9712 Level II or ASNT Level II minimum, documented. Welds listed in the critical weld table (sheet 2) to be inspected by the method and extent stated, by an independent third-party agency, after welding and before stress relief. Acceptance: ISO 5817 level B for critical welds, level C elsewhere. Reports, including operator qualification records, to be supplied with the part.

Roughly seventy words, and it removes essentially every NDT argument that normally happens.

If you have a frame drawing and want help deciding which welds belong on the critical list before it goes out to quote, send it through — the review comes back within 24 business hours.