A welded machine base, a skid frame or a process tank is usually the largest single item in a project, and the one that arrives last. When something is wrong with it, the cost is out of proportion to its price: a frame that is out of square holds up the installation of every machine that sits on it, and a tank with a weld leak cannot simply be sent back across the sea.
This article is for engineers and buyers sending welded fabrications — frames, bases, platforms, guards, hoppers, non-pressure tanks — to a Vietnamese fabricator. It covers the welding system to ask for, what the drawing must say, coating, dimensional inspection after welding, and the question that should be asked before any drawing is finished: how big can the piece be and still be shipped. Sheet-metal enclosures and light welded sheet parts are covered in sheet metal and stamping in Vietnam.
💬 Not sure whether your fabrication should ship in one piece or in modules? Send us the general arrangement drawing and we will tell you what a Vietnamese fabricator and forwarder will ask first.
Ask for a welding system, not a welder
Weld quality comes from a controlled system — qualified procedures, qualified welders, a welding coordinator and records — and the ISO 3834 series is the common way to name the level you need.
The ISO 3834 series sets quality requirements for fusion welding of metallic materials in the workshop and on site. Part 2 (comprehensive requirements, current edition 2021), Part 3 (standard requirements) and Part 4 (elementary requirements) give three levels; Part 5 lists the documents needed to claim conformity. A fabricator may or may not hold third-party certification to one of these parts. If it does not, the same questions still apply — they are simply answered by inspection instead of by certificate.
| Element | Standard to name | What to ask the fabricator for |
|---|---|---|
| Welding procedure specification (WPS) | ISO 15609-1:2019 for arc welding | The WPS for each joint type on your drawing |
| Procedure qualification (WPQR) | ISO 15614-1:2017 (with Amendment 1:2019), level 1 or 2 | The qualification record behind each WPS |
| Welder qualification | ISO 9606-1 for steels | Certificates for the welders who will weld your job |
| Welding operator qualification | ISO 14732:2025 for mechanised and automatic welding | Certificates if robots or carriages are used |
| Quality level of the system | ISO 3834-2, -3 or -4 | Certificate, or evidence of each element on a visit |
| North American projects | AWS D1.1/D1.1M:2025, Structural Welding Code – Steel | Procedures and welder qualifications to that code |
Two practical points come up on almost every first job.
A welder certificate covers a range, not everything. Under ISO 9606-1 a welder is qualified for a process, a material group, a type of joint, positions and a thickness and diameter range. A welder qualified flat on plate may not be qualified for overhead pipe welds. Match the certificates to the joints on your drawing, and ask whether the certified welders are the ones who will actually weld your job.
ISO 15614-1 has two levels. Level 2 involves more testing and narrower ranges of qualification than level 1, and applies when the contract does not name a level. Say which you require.
If the fabrication will be placed on the market as a structural steel product in a jurisdiction with its own execution rules — for example execution classes under EN 1090-2 in the EU — the designer sets the requirement, and the fabricator must be able to meet it. Confirm that with your own engineer and the destination’s current rules before ordering.
What the drawing must say about welds
Every weld needs a symbol with size, length and position, plus a quality level for imperfections and a statement of which welds are tested and how.
Weld symbols. ISO 2553:2019 sets the rules for weld symbols on drawings. It contains two systems for showing the arrow side and the other side — one with a dual reference line and one with a single reference line — so state which system your drawing follows, and whether it follows a North American symbol convention instead. ISO 2553 also allows a simpler designation in which the drawing gives the weld dimensions and quality level and leaves joint preparation to the fabricator; that works only if the fabricator’s WPS is reviewed.
Quality level. ISO 5817:2023 gives three quality levels — B, C and D, with B the strictest — for imperfections in fusion-welded joints in steel, nickel, titanium and their alloys. Naming a level per weld or per drawing turns “good welding” into acceptance limits for porosity, undercut, lack of fusion, excess reinforcement and the rest. Not every weld needs level B; demanding it everywhere raises cost without improving a frame’s function.
Testing scope. Visual inspection of every weld is the baseline. Then name which welds receive surface testing (magnetic particle or penetrant) and which receive volumetric testing (ultrasonic or radiography), at which extent, and to which acceptance standard. “NDT as required” means nothing at quotation stage.
Repairs. State that weld repairs follow a qualified procedure and are recorded, and that repairs on critical welds need your approval.
💬 We can review a fabrication drawing for what a Vietnamese shop will query — weld symbols, quality levels, testing scope and transport splits — and run the quotation round on one basis, see sourcing from Vietnam.
Dimensional tolerances after welding
Welding distorts steel, so the drawing needs general tolerances for welded constructions, the measuring method, and a decision on which faces are machined after welding.
ISO 13920:2023 gives four classes of general tolerances for linear and angular dimensions and for shape and position of welded constructions. Under that standard, a manufacturing drawing that gives dimensions without individual tolerances and without a reference to general tolerances is considered incomplete. Write the class in the title block, and give individual tolerances where function needs them — mounting pads, bearing seats, guide rails.
Where a machine sits on a frame, flatness and the relative position of mounting faces usually matter more than overall length. The usual answer is to weld first, stress relieve if the design needs it, and machine the mounting faces afterwards. Ask the fabricator whether it machines in-house or sends the frame to a machine shop large enough for it, and who answers for distortion found at machining.
Agree how the frame is measured: on what surface, with which instruments, and at what stage — after welding, after machining, after coating. Hot-dip galvanising and stress relieving can move a frame, so measure after the last thermal step. Ask for a dimensional report with the measuring points marked on the drawing.
Surface preparation and coating
The coating specification must name the surface preparation grade, the coating system for the environment and the thickness check — and, for galvanising, the design details the process needs.
Paint systems. ISO 8501-1 defines rust grades and preparation grades of steel surfaces; blast-cleaning grades carry the “Sa” designation, such as Sa 2½. The ISO 12944 series classifies atmospheric environments into corrosivity categories from C1 to CX and links them to protective paint systems. On the specification, write the preparation grade, the corrosivity category of the place where the steel will stand, the paint system (primer, intermediate, top coat), the dry film thickness and how it is measured, and the colour reference. Vietnam’s climate is humid, so ask how blasted steel is protected before priming and how long it may wait.
Hot-dip galvanising. ISO 1461:2022 covers hot-dip galvanised coatings on fabricated iron and steel articles. Galvanising needs vent and drain holes in hollow sections and closed spaces, and the piece has to fit the galvaniser’s bath. In Vietnam the galvanising is often done by a separate galvanising plant, so ask which plant, what bath size it has, and who transports and inspects the parts. A frame designed without that information can end up double-dipped or redesigned.
Masking. Name the areas that stay bare: machined faces, earthing points, threaded holes, faying surfaces of bolted connections.
Tanks: leak tests and pressure rules
A tank needs a defined leak or hydrostatic test, an inside-surface specification and, if it holds pressure, a decision on which pressure-equipment rules apply at the place of use.
For a non-pressure tank, specify the test: water fill and hold, or a penetrant or vacuum-box test on seams, with the hold time and acceptance criterion agreed in writing. Specify the inside finish — weld grinding, pickling and passivation for stainless — if the tank holds food, chemicals or water for process use.
If the vessel will operate under pressure, its design, manufacture and inspection fall under the pressure-equipment rules of the country where it will be installed, which differ by country and by size and pressure. Settle which code applies before asking for a price, and confirm it against the current rules of the place of use. Steel tanks of more than 300 litres for materials other than compressed or liquefied gas fall under HS heading 7309, and smaller ones under 7310; confirm the full national code with your broker.
How big can it be and still ship?
The transport size is a design input, so decide early whether the piece ships in a standard container, on special equipment or in modules, and design the bolted splits accordingly.
The route is roughly: standard containers for anything that fits through the door with lifting points accessible; open-top or flat-rack containers for pieces that are too tall or too wide; break-bulk shipment for the largest items. Each step up adds cost and handling risk. Ask the forwarder for current internal and door dimensions of the equipment available on your route rather than relying on a general table.
Inland transport in Vietnam has its own limits. Loads exceeding the road size or weight limits need a circulation permit under the road-load rules; these were set out in Circular 39/2024/TT-BGTVT, and the Ministry of Construction has since issued Circular 12/2025/TT-BXD on the same subject. Confirm the current text and allow time for the permit if the piece is oversized. Port charges for handling depend on the Incoterm; see Vietnam local charges by Incoterm.
If the piece is too big, split it into modules with bolted joints, and ask the fabricator to trial-assemble the whole structure in the workshop before dismantling it, with match marks on every joint and the bolt sets packed per joint. A frame that is never assembled until it reaches site will reveal its errors there.
Pack for the sea: protect machined faces with rust preventive, add lifting and lashing points to the design, mark the centre of gravity and weight on large pieces, and photograph the load before closing. Export shipping documents from Vietnam covers the paperwork side.
| Decision | Ask before the drawing is final | Cost of deciding late |
|---|---|---|
| Welding system | ISO 3834 level, WPS, WPQR, welder certificates | Re-welding or re-qualification |
| Weld quality and testing | ISO 5817 level, NDT method and extent | Disputes at final inspection |
| Tolerances | ISO 13920 class, critical dimensions, machining after welding | Frames that do not fit the machines |
| Coating | Sa grade, corrosivity category, system, thickness or galvanising | Early rust, or redesign for galvanising |
| Tank test | Test method, hold, acceptance, pressure code | Leaks found at site |
| Transport | Shipping mode, module split, trial assembly | Pieces that cannot be moved |
General points on a first visit are in what to check on a first visit to a Vietnamese factory. Bolts for the field joints are a purchase of their own; see fasteners and springs from Vietnam. Forged lifting eyes and flanges are covered in forgings from Vietnam.
💬 Send us the drawings even before the specification is complete. For each project we find, vet and manage the fabricator that fits the job, witness trial assembly and arrange export. Get in touch.
Frequently asked questions
1. Does a Vietnamese fabricator need ISO 3834 certification?
Not necessarily. What you need is the system the standard describes: qualified procedures, qualified welders, coordination and records. Certification is evidence that the system exists; without it, check each element on a visit and in the documents.
2. Are welder certificates enough to prove weld quality?
No. A certificate shows a welder passed a test within a defined range. Weld quality on your job also depends on the qualified procedure, the joint preparation and the inspection. Ask for the WPS and the record that qualifies it.
3. Which weld quality level should I specify?
Choose per weld by function. ISO 5817 gives levels B, C and D, with B the strictest. Highly loaded or fatigue-sensitive welds may justify a stricter level; secondary welds usually do not.
4. How do I stop a frame from arriving out of square?
Give an ISO 13920 tolerance class plus individual tolerances on critical faces, machine mounting faces after welding, measure after the last thermal process, and require a dimensional report with marked measuring points.
5. My structure is too big for a container. What are the options?
Open-top or flat-rack containers, break-bulk shipment, or splitting into bolted modules. Splitting is often the simplest, provided the joints are designed in and the structure is trial-assembled and match-marked in the workshop.
Sources: ISO 3834-2:2021 and the ISO 3834 series; ISO 15609-1:2019; ISO 15614-1:2017 with Amd 1:2019; ISO 9606-1; ISO 14732:2025; AWS D1.1/D1.1M:2025; ISO 2553:2019; ISO 5817:2023; ISO 13920:2023; ISO 8501-1:2007; ISO 12944 series; ISO 1461:2022; EN 1090-2; Circular 39/2024/TT-BGTVT; Circular 12/2025/TT-BXD; Harmonized System headings 7309 and 7310. Checked on 19 October 2026.
More industrial sourcing guides: Fasteners and Springs From Vietnam: Classes, Coatings, Lots · Aluminium Extrusion in Vietnam: Alloy, Die, Finish and Cutting · Jigs, Fixtures and Automation Machines Built in Vietnam. All guides by category are on the industrial sourcing page.