Welding in Structural Steel — Part 2: Procedures, Inspection and Practical Tips

Before you start

Part 1 answers the question “what do I put on the drawing?”. Part 2 answers the harder one: “how do I know the weld in the field matches the drawing?” — from procedure records and welder qualification through to inspection and acceptance.

1. Welding procedures — WPS, PQR and WPQ

Abbreviation	Full name	Role
WPS	Welding Procedure Specification	“The recipe” — detailed instructions on how to weld
PQR	Procedure Qualification Record	“The test” — proves the WPS actually works
WPQ	Welder Performance Qualification	“The licence” — certifies the welder is capable

Figure 6. The relationship between WPS, PQR and WPQ. — **Figure 6.** The relationship between WPS, PQR and WPQ.

Prequalified WPS — the legal shortcut

AWS D1.1 allows a prequalified WPS without PQR testing, provided ALL five conditions are met:

Approved processes: SMAW, GMAW (spray transfer only), FCAW, SAW. GMAW short-circuit is not prequalifiable.
Base metal in a prequalified group (Groups I–IV, Table 5.3).
Matching filler metal per Table 5.4.
Joint details complying with the prequalified figures (5.1–5.4).
Parameters within limits: preheat, interpass temperature, current, voltage and travel speed.

A PQR is required when: the joint is not in the prequalified list; the process is outside those four (EGW, ESW…); parameters exceed the limits; the base metal is not in a prequalified group; or the design engineer requires it.

2. Weld inspection and acceptance

Method	Abbrev.	Detects	When to use
Visual inspection	VT	Surface defects	Mandatory for all welds
Ultrasonic testing	UT	Internal defects	CJP groove welds, thick plates
Radiographic testing	RT	Internal defects (image)	CJP groove welds, alternative to UT
Magnetic particle	MT	Surface and near-surface	Ferromagnetic materials (carbon steel)
Liquid penetrant	PT	Open surface defects	Non-magnetic materials (stainless)

Figure 7. NDT methods and the inspection sequence. — **Figure 7.** NDT methods and the inspection sequence.

Visual acceptance criteria (AWS D1.1, Clause 8)

Defect	Allowable limit
Crack	Not acceptable — any size
Overlap	Not acceptable
Surface porosity	CJP in transverse tension: no piping porosity allowed
Undercut	t < 25 mm: ≤ 1 mm · t ≥ 25 mm: ≤ 2 mm
Undersized or short	Not acceptable

For ultrasonic testing, AWS D1.1 classifies indications as Class A–D based on signal amplitude and indication length: Class A is a severe defect and is rejected, Class D is a small acceptable indication — see Table 8.2. For radiography, the key advantage is the permanent record on film or in digital form.

Figure 8. Six common weld defects and their acceptance criteria. — **Figure 8.** Six common weld defects and their acceptance criteria.

3. The ten most common errors and how to avoid them

Design errors

Error	Consequence	Prevention
No weld size specified	The welder decides; no quality control	Always note size and length on the symbol
Arrow side and other side reversed	Weld on the wrong side	Review drawings against a checklist
CJP specified without backing information	Disputes on site and delays	Always state whether backing is used
Ignoring the min/max size limits	Code violation and rework	Use AISC 360 Table J2.4
No shop or field weld distinction	The erector does not know the sequence	Use the field weld symbol (⚑)

Fabrication and erection errors

Error	Consequence	Prevention
No surface cleaning	Porosity and lack of fusion	A mandatory cleaning procedure
Welding by feel	Unstable parameters and a high reject rate	Follow the WPS and check before each weld
Uneven preheat	Hydrogen cracking and a hard HAZ	Use controlled heating equipment
Wrong filler metal	A weld weaker than designed	Verify the filler metal before issue
Poor fit-up	Root opening too large or too small → lack of fusion or distortion	Use gauges and comply with tolerances

4. Practical tips for design engineers

The KISS principle — Keep It Simple, Structural

Prefer fillet welds whenever possible — cheaper, easier and faster.
Use CJP only when full member strength is genuinely required.
PJP is a good mid-range option for compression connections.
Avoid over-welding — it wastes material and increases distortion and residual stress.

Weld cost optimisation

Fillet weld cost scales with the square of the leg size: doubling the size means four times the filler material. When more capacity is needed, increase the length rather than the size.

One side:  w = 12 mm    →  A = 0.5 × 12²     = 72 mm²
Two sides: w = 8 mm × 2 →  A = 2 × (0.5 × 8²) = 64 mm²
→ about 11% material saved, equal or better capacity

Special attention for seismic design

AISC 341 is stricter than AISC 360; CJP is mandatory for beam-flange-to-column connections in moment frames.
Demand critical welds require: filler metal passing the CVN toughness test, a WPS qualified by PQR, and 100% NDT.
Protected zone — no attachments or welding are permitted within it.

5. Related standards

Standard	Scope	What engineers need to know
AISC 360	Structural steel design	Chapter J — welded connection design requirements
AISC 341	Seismic design	Demand critical welds, protected zone
AWS D1.1	Structural steel welding	WPS/PQR, prequalified procedures, inspection, acceptance
AWS A2.4	Welding symbols on drawings	How to read and write welding symbols
AWS D1.8	Seismic welding supplement	Supplements D1.1 for seismic applications
ASTM A6	Structural steel shapes	Tolerances and mechanical properties
ASTM A36, A992, A572	Common steel grades	Strength and weldability

6. Comprehensive project checklist

Design phase

Appropriate weld type selected for each connection?
Weld size and length calculated?
Min/max size verified per AISC 360 Table J2.4?
Complete welding symbols per AWS A2.4?
Shop and field welds clearly distinguished?
NDT requirements specified (VT, UT, RT, MT, PT)?
Demand critical welds identified where applicable?

Fabrication phase (shop)

WPS prepared and approved?
Welder holds a valid WPQ?
Base metal and filler metal verified?
Fit-up within tolerance?
Preheat and interpass temperatures per the WPS?
Visual inspection after each pass for multi-pass CJP?
NDT performed upon completion?

Erection phase (field)

Field welds have their own WPS?
Environmental conditions acceptable (wind, rain, temperature)?
Field welders hold a WPQ for the required positions?
NDT carried out at the specified rate?
Acceptance records complete?

Closing

A weld is not just a line of molten metal on a drawing. It is the language of communication between the design engineer, the fabricator, the welder and the inspector. Understand correctly, specify correctly, inspect correctly — that is a safe structure.

Key point	Keyword
Fillet is the most common; throat = 0.707 × leg	te = 0.707w
CJP develops full strength, PJP only part of it	CJP vs PJP
Below the reference line is arrow side, above is other side	Arrow / Other
Minimum size follows the thinner plate	Table J2.4
No WPS means no welding	No WPS = No Weld
Visual inspection is mandatory on 100% of welds	VT = Gatekeeper
A crack is an immediate reject, in every case	Zero Crack
High-strength steel waits 48 hours before inspection	48h Rule
Weld cost scales with the square of the leg size	Cost ∝ w²
Demand critical welds need their own PQR	No Prequalified for DCW

Part of the series "Structural design for industrial facilities" — Roberto Structural. The content is technical guidance; the engineer remains responsible for checking and adapting it to the conditions of each project and the requirements of the governing code. Standards evolve — always check the latest editions of AWS D1.1, AISC 360 and AWS A2.4 before applying them.