ISO 5817 quality levels B, C, D: imperfection table

EN ISO 5817 (PN-EN ISO 5817 in Poland) specifies how large welding imperfections, such as pores, undercut, excess weld metal, lack of fusion and incomplete penetration, may be in welded joints in steel, nickel, titanium and their alloys. It has three quality levels: B, C and D; the strictest is B. You need it when you manufacture structures to EN 1090-2, assess welds visually yourself, or prepare for a test or an audit.

Current edition and scope of the standard

The current edition is the fourth, ISO 5817:2023, published in February 2023 (26 pages). PKN (the Polish Committee for Standardization) introduced it in August 2023 as PN-EN ISO 5817:2023-08 in the English language version. It replaced PN-EN ISO 5817:2014-05, which also had a Polish version.

The standard covers:

non-alloy and alloy steels, nickel and its alloys, titanium and its alloys;
material with a thickness of 0.5 mm or more;
full penetration butt welds and all fillet welds; its principles can be transferred to partial penetration butt welds;
manual, mechanised and automatic welding in all positions;
processes according to ISO 4063: 11 (metal arc welding without gas protection, for example covered electrode), 12 (submerged arc), 13 (MIG/MAG), 14 (TIG), 15 (plasma) and 31 (oxy-fuel gas welding, for steel only).

The standard does not cover:

joints welded by electron beam or laser; for steel the quality levels are given in ISO 13919-1;
aluminium and its alloys: for these there is a separate standard, ISO 10042:2018, also with levels B, C and D, for MIG, TIG and plasma welding;
metallurgical characteristics, for example grain size and hardness.

The 2023 edition changed, among other things, the criteria for multiple imperfections (item 4.1 of the table) and some of the figures, and in several formulas it introduced the actual fillet weld throat thickness aA. Additional criteria for welds subject to fatigue loading are in Annex B.

What levels B, C and D mean

All three levels describe the quality of workmanship in production, not the fitness of the weld for a specific application. The limits are given as formulas based on the dimensions of the joint: plate thickness t, nominal butt weld thickness s, fillet weld throat thickness a and width of the excess weld metal b. That is why an undercut 0.9 mm deep will pass at level D on a 10 mm plate, but not on a 4 mm plate.

General rules:

The level is assigned to the joint, not to the whole product. In one structure different joints may have different levels, and sometimes different levels for different imperfections in the same joint.
A short imperfection has a total length of up to 25 mm in the 100 mm section of weld where there are the most imperfections (in welds shorter than 100 mm, up to 25% of their length).
Systematic imperfections, repeated along the whole length of the weld, are permitted only at level D.
Two adjacent imperfections separated by less than the largest dimension of the smaller one are assessed as one.
Level B may require additional operations, for example grinding or TIG dressing of the weld toe.

Who chooses the quality level

The level is specified by the product standard or by the responsible designer in agreement with the manufacturer, the user and other parties. The standard requires it to be set before production starts, ideally already at the enquiry or order stage. The choice depends on the design, subsequent processing, type of loading (static, dynamic), service conditions and consequences of failure. The standard also recommends taking into account the costs of welding, inspection, testing and repairs.

Quality level and EN 1090-2 execution classes

In steel structures the level usually follows from the execution class EXC. According to EN 1090-2:2018:

EXC1: level D, but for insufficient throat thickness (5213) level C;
EXC2: level C, although for overlap (506), arc strike (601) and end crater pipe (2025) D is sufficient, and for insufficient throat thickness (5213) B is required;
EXC3: level B;
EXC4: at least the requirements of EXC3 plus additional requirements set for specific welds.

The execution class is specified by the designer in the documentation. The 2018 edition no longer provides for a default class EXC2 when no class has been given. In Poland the current edition is PN-EN 1090-2+A1:2024-10 (English version), which replaced PN-EN 1090-2:2018-09. Amendment A1 changed the weld acceptance criteria for EXC1 and EXC2, so check the list of exceptions in the edition specified in the contract.

Table: selected imperfections and their limits

The values come from Table 1 of ISO 5817:2023, which has more than 40 items with figures and notes; for acceptance use the full text of the standard. Symbols: t plate or wall thickness, s nominal butt weld thickness, a fillet weld throat thickness, b width of excess weld metal, h height of the imperfection, d pore diameter.

No. per ISO 6520-1	Imperfection	Level D	Level C	Level B
100	Crack	not permitted	not permitted	not permitted
2017	Surface pore, butt weld, t > 3 mm	d ≤ 0.3 s, max. 3 mm	d ≤ 0.2 s, max. 2 mm	not permitted
2011, 2012	Gas pores and porosity, multi-layer weld, proportion of projected area	≤ 5%	≤ 3%	≤ 2%
401	Internal lack of fusion, butt weld	short, h ≤ 0.4 s, max. 4 mm	not permitted	not permitted
4021	Incomplete root penetration, single-side weld with full penetration required	short, h ≤ 0.2 t, max. 2 mm	not permitted	not permitted
5011, 5012	Continuous and intermittent undercut, t > 3 mm	h ≤ 0.2 t, max. 1 mm	h ≤ 0.1 t, max. 0.5 mm	h ≤ 0.05 t, max. 0.5 mm
502	Excess weld metal on the face of a butt weld	h ≤ 1 mm + 0.25 b, max. 10 mm	h ≤ 1 mm + 0.15 b, max. 7 mm	h ≤ 1 mm + 0.1 b, max. 5 mm
503	Excessive convexity of a fillet weld	h ≤ 1 mm + 0.25 b, max. 5 mm	h ≤ 1 mm + 0.15 b, max. 4 mm	h ≤ 1 mm + 0.1 b, max. 3 mm
504	Excessive penetration, t > 3 mm	h ≤ 1 mm + 1.0 b, max. 5 mm	h ≤ 1 mm + 0.45 b, max. 4 mm	h ≤ 1 mm + 0.2 b, max. 3 mm
506	Overlap	h ≤ 0.2 b	not permitted	not permitted
601	Arc strike	permitted if it does not change the properties of the parent metal	not permitted	not permitted

Lack of fusion visible on the weld surface is not permitted at any of the three levels. For plates from 0.5 to 3 mm the limits differ from those in the table: for example, undercut is not permitted there at level B, and a surface pore at C and B. The standard makes the acceptance of spatter (602) depend on the application, for example the material and corrosion protection.

Worked example

You have a 6 mm plate. The undercut may be at most: at level D 1 mm (0.2 t gives 1.2 mm, but the 1 mm maximum applies), at C 0.5 mm (0.1 t gives 0.6 mm, max. 0.5 mm), at B 0.3 mm. The face of the butt weld is 10 mm wide. The excess weld metal on the face may be at most 3.5 mm at level D, 2.5 mm at C and 2 mm at B.

Visual and non-destructive testing

In visual testing you assess welds directly in accordance with ISO 5817. How the testing is carried out is described in ISO 17637:2016 (PN-EN ISO 17637:2017-02 in Poland). It requires surface illumination of at least 350 lx (500 lx recommended) and access such that the eye is no further than 600 mm from the surface and views it at an angle of at least 30°. When this is not possible, you use mirrors, endoscopes or cameras. ISO 17637 does not set the extent of visual testing: it follows from the product standard or the contract.

ISO 5817 does not describe methods for detecting internal imperfections. Non-destructive testing has its own standards with acceptance levels, and their relationship with the quality levels is given in ISO 17635 (2025 edition, in Poland PN-EN ISO 17635:2025-10). An acceptance level corresponds to a quality level only in general terms, for a whole batch of welds, and not for each individual indication.

Radiography, ISO 10675-1:2021: level B corresponds to acceptance level 1, level C to level 2, level D to level 3. For B and C, radiographic technique class B is used, for D class A.
Ultrasonic testing, ISO 11666:2018 (full penetration welds in ferritic steels, thickness 8 to 100 mm): acceptance level 2 corresponds to level B, level 3 to level C. For D the standard gives no acceptance level, because UT is not normally required then.

Typical mistakes in applying the standard

Level set only at acceptance. The customer assesses the welds to B, although the order did not state a level. The standard requires it to be set before production.
Level B just in case. That means more grinding, rework and testing. Match the level to the joint.
Confusing quality level with acceptance level. B, C and D are ISO 5817 quality levels. Radiography has acceptance levels 1, 2 and 3, and ultrasonic testing 2 and 3.
Assessing aluminium or laser welds to ISO 5817. For aluminium the right standard is ISO 10042, for beam welding ISO 13919-1.
Assessing by eye. Without a weld gauge, light and the dimensions t, s and a, you cannot calculate the limit, because the standard gives formulas.
No edition year in the documents. The 2014 and 2023 editions differ, among other things, in the criteria for multiple imperfections. In older drawings you may also come across the designation B+ for EXC4 from the earlier edition of EN 1090-2; check what applies to the order.

What to do step by step in a small company

In the order, state the standard with its year (EN ISO 5817:2023, PN-EN ISO 5817:2023-08 in Poland), the quality level for each type of joint or the execution class EXC, the extent of visual and non-destructive testing, and the acceptance level standards.
Take the thicknesses t, s and a and the face width from the drawing, calculate the limits for the most common imperfections and give the welders a sheet with the results.
Set up an inspection station: a lamp giving at least 350 lx on the weld, a weld gauge, a magnifier with 2 to 5 times magnification, a mirror, a report template.
Inspect welds right after they are made, while there is still access to them. Record imperfections by their ISO 6520-1 numbers (for example 5011, 502), with size and location.
Look for the causes of recurring imperfections. Pores, lack of fusion and spatter often result from the gas shielding, edge preparation, parameters or the condition of the equipment. If the arc is unstable at the same settings or the feeder feeds the wire unevenly, send the welding machine for servicing or consider replacing it with a used welding machine.
For welds subject to fatigue loading, check whether the designer has entered a level from Annex B, for example ISO 5817-C63, ISO 5817-B90 or ISO 5817-B125. Level B125 is usually not achievable without post-weld treatment and does not cover fillet welds, and Annex B does not apply to level D.

Frequently asked questions

Which level is the highest: B or D?

The highest is B, the most lenient D. Level C lies in the middle and is the basic level for class EXC2.

Does EN ISO 5817 apply to aluminium?

No. Joints in aluminium and its alloys are assessed in accordance with ISO 10042:2018, which has the same designations B, C and D but its own table of limits.

Which quality level applies to EXC2?

According to EN 1090-2:2018, level C, with the exceptions for a few imperfections described above. The execution class must be specified by the designer in the documentation.

Are cracks permitted at level D?

No. Cracks (100) and crater cracks (104) are not permitted at any level. The standard deals separately with microcracks, which are visible only under a microscope at a magnification of at least 50 times: at D they are permitted, at C and B the assessment depends on the material and its susceptibility to cracking.

Does level B mean mandatory radiography?

No. ISO 5817 does not set the methods or the extent of testing. The extent of non-destructive testing is specified by the product standard, for example EN 1090-2, or by the contract.

EN ISO 5817 quality levels B, C and D: table and practical use