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newSchoolWorks.com › VoTechWorks › Building & Construction › Welding › Capstone Project: Structural and Pipe Weldment Fabrication › Lesson 3
BLD-WELD01-12 · Lesson 3 · 96 clock hours · practice

Multi-Process Welding, In-Process Inspection, and Distortion Control

Objective. Students complete all full-penetration groove welds and fillet welds on the capstone assembly using at least three welding processes (SMAW, GMAW or FCAW, and GTAW for pipe root or thin-section joints) in accordance with the approved WPS, maintaining a written in-process inspection log that records interpass temperature, interpass cleaning, and visual findings after each pass.

Multi-Process Welding, In-Process Inspection, and Distortion Control

This capstone lesson brings together every welding process, inspection skill, and fabrication strategy you have practiced. By the end, you will have deposited full-penetration groove welds and fillet welds using SMAW, GMAW or FCAW, and GTAW on a single structural and pipe assembly — all in strict conformance with your approved Welding Procedure Specification (WPS).

Safety First

Before striking an arc on any process, confirm your PPE is correct for that process. Auto-darkening helmets must be set to the shade required by the current process and amperage (GTAW typically demands a darker shade than low-amperage GMAW). Leather gloves, jacket or sleeves, and leather boots are required at all times. When grinding between passes, add a face shield over your safety glasses. Review the SDS for each filler metal and shielding gas in use; low-hydrogen electrodes (E7018), flux-core wire (E71T-1), and argon-shielded GTAW each present different fume profiles. Use local exhaust ventilation or a respirator appropriate to the fume hazard identified in the SDS. Apply lockout/tagout before changing wire spools, swapping torch components, or clearing wire-feed jams. Never leave a live GTAW torch unattended with the high-frequency start enabled.

Following the WPS — Essential Variables

The WPS is not a suggestion; it is the document your welder qualification is tied to. Essential variables — amperage range, voltage, travel speed, filler metal classification, shielding gas composition, and preheat/interpass temperature limits — cannot be changed in the field without engineer approval and a new or revised procedure. Work to the midpoint of each range when conditions are favorable, and adjust only within the listed limits. Record the actual parameters you use in the inspection log alongside each pass entry.

Preheat and Interpass Temperature

Preheat drives moisture from base metal and slows the cooling rate, reducing the risk of hydrogen-induced cracking and hard heat-affected zones. Interpass temperature is the temperature of the weld joint measured immediately before a new pass is deposited. Both limits are specified in the WPS and backed by AWS D1.1 requirements for the base metal group.

Use a contact pyrometer or infrared thermometer to check temperature at a point about one inch from the joint edge. If the interpass temperature exceeds the WPS maximum (commonly 400–600 °F for structural carbon steel, but always use your specific WPS value), stop welding, allow the assembly to cool naturally — do not quench with water — and re-verify before continuing. Log every temperature reading with the time, joint ID, and pass number. A complete log is not optional; it is evidence that the weld was produced in conformance with the qualified procedure.

In-Process Visual Inspection After Each Pass

For SMAW and FCAW, chip slag thoroughly with a chipping hammer, then wire-brush the bead until bare metal is visible across the full width. Do not skip cleaning at the toes of the bead where slag traps commonly hide. Once clean, inspect visually using good lighting:

Record every finding in the log: location by joint ID and pass number, type of discontinuity, dimensions, and disposition (accept per code criteria, or grind and re-weld). A clean bead on the previous pass is the foundation for a sound pass on top of it.

Sequencing Joints and Controlling Distortion

Heat causes metal to expand; restrained cooling causes it to contract and distort. On a multi-joint assembly, poor sequencing multiplies distortion. Use the following strategies:

At defined hold points — after root passes, after fill passes, and after cap passes on each joint — measure squareness of structural members with a square and tape, and check pipe alignment against the drawing. Document every corrective action taken.

Using GTAW for Root and Thin-Section Joints

GTAW (TIG) is used for the pipe root pass and any thin-section joint in your assembly. The process requires the highest skill: one hand manipulates the torch and the other feeds filler rod. Argon shielding gas protects the tungsten electrode and the weld pool. High-frequency start ignites the arc without touching the tungsten to the base metal, preventing contamination. Keep the tungsten sharp and clean; a contaminated tungsten causes arc wander and porosity. Use ER70S-2 filler rod for carbon steel pipe root passes — its deoxidizer package tolerates minor surface oxidation and produces a clean, sound root bead from the outside without a backing strip.

Hold-Point Inspection and Final Visual Inspection

Your instructor will perform a mid-project hold-point review of all root passes and the inspection log before fill passes begin. Treat this as a quality gate: unacceptable root passes must be repaired before fill welding covers them. At project completion, your instructor will conduct a formal visual weld inspection (VT) using a weld gauge and tape measure, scoring against a 25-point rubric. A score of 80 % or higher, with zero cracks and zero unrepaired undercut exceeding D1.1 limits, is required to proceed to the documentation and defense phase. Approach every pass as though it is the pass that will be measured — because any of them might be.

Key terms10

Welding Procedure Specification (WPS)A written document that prescribes the essential variables — filler metal, amperage, voltage, travel speed, preheat, and interpass temperature — required to produce a weld meeting code requirements; welders must not deviate from its essential variables.
Essential variableAny WPS parameter whose change requires re-qualification of the procedure, such as base metal group, filler metal classification, shielding gas, or preheat temperature; defined by AWS D1.1 and ASME Section IX.
Interpass temperatureThe temperature of the weld joint immediately before a new pass is deposited; must remain within the minimum (preheat) and maximum limits stated in the WPS to control cooling rate and avoid cracking or loss of toughness.
Back-step sequencingA distortion control technique in which short weld segments are deposited in the direction opposite the overall weld progression, so that shrinkage forces in adjacent segments partially cancel each other.
In-process inspection logA written record completed during fabrication that documents preheat and interpass temperatures, interpass cleaning actions, and visual findings after each pass, providing traceability that the weld was made per the qualified procedure.
UndercutA groove melted into the base metal at the toe of a weld bead and left unfilled; it acts as a stress concentration and is subject to strict depth and length limits under AWS D1.1 visual acceptance criteria.
Full-penetration groove weldA groove weld in which fusion extends through the entire thickness of the joint; required for structural connections carrying primary load and evaluated for complete fusion by visual, ultrasonic, or radiographic testing.
Heat inputThe amount of thermal energy delivered per unit length of weld, calculated from voltage, amperage, and travel speed; higher heat input increases grain growth in the heat-affected zone and increases weld shrinkage and distortion.
High-frequency start (GTAW)An electrical feature on GTAW power sources that initiates the arc with a high-frequency, high-voltage spark so the tungsten electrode never touches the base metal, preventing tungsten contamination of the weld pool.
Hold-point inspectionA planned stop in fabrication at which work cannot continue until an inspector or instructor reviews and approves the work completed to that point; used to catch defects before they are buried under subsequent passes.

Worked examples4

1. Your WPS specifies a maximum interpass temperature of 450 °F. You measure 380 °F before starting the next fill pass. Is it acceptable to proceed? Show your comparison.
  1. Identify the WPS maximum interpass temperature: 450 °F.
  2. Record the measured temperature: 380 °F.
  3. Compare: 380 °F ≤ 450 °F.
  4. Also confirm the measured temperature meets the minimum preheat requirement listed in the WPS (for example, 150 °F minimum for low-hydrogen process on this base metal group); 380 °F ≥ 150 °F, so the minimum is also satisfied.
Answer: Yes — 380 °F is within the WPS interpass window (at or above minimum preheat, and below the 450 °F maximum). Record '380 °F — acceptable' in the inspection log and proceed with the next pass.
2. After completing a fill pass you measure an interpass temperature of 510 °F. Your WPS maximum is 450 °F. How long must you wait if the joint cools at approximately 15 °F per minute, and what temperature must you confirm before continuing?
  1. Determine the excess temperature: 510 °F − 450 °F = 60 °F above the maximum.
  2. At a cooling rate of 15 °F per minute, estimate minimum wait time: 60 °F ÷ 15 °F/min = 4 minutes.
  3. After 4 minutes, re-verify with the thermometer. Do not rely on the estimate alone — actual cooling rate varies with ambient conditions and joint mass.
  4. The temperature you must confirm before continuing is at or below 450 °F and at or above the WPS minimum preheat, confirming the joint is within the acceptable interpass window.
Answer: Wait at least 4 minutes (estimate), then re-measure. Proceed only when the thermometer reads 450 °F or below and at or above the minimum preheat. Record the re-verified temperature in the log.
3. A weld gauge measurement shows undercut at the toe of a fillet weld is 1/32 inch (approximately 0.8 mm) deep and 3/4 inch long. AWS D1.1 visual acceptance criteria for statically loaded structures allow undercut no deeper than 1/16 inch (approximately 1.6 mm) for any length. Is this undercut acceptable or rejectable by visual criteria?
  1. Identify the measured undercut depth: 1/32 inch = 0.031 inch.
  2. Identify the D1.1 allowable depth for statically loaded members: 1/16 inch = 0.063 inch.
  3. Compare depth: 0.031 inch ≤ 0.063 inch — depth is within the allowable limit.
  4. Note: D1.1 also applies stricter limits for cyclically loaded structures and for undercut parallel to primary stress; confirm the loading category in the project drawings. For this statically loaded example, the depth criterion is met.
  5. Record the finding: '3/4 in. undercut at toe, 1/32 in. deep — accept per D1.1 visual criteria for static loading.' Note the location and pass in the inspection log.
Answer: Acceptable for a statically loaded connection — the 1/32 inch depth is within the 1/16 inch D1.1 limit. Document the finding and disposition in the inspection log. Always verify the loading category; cyclically loaded joints have stricter limits.
4. You are back-step sequencing a 12-inch fillet weld using 3-inch increments to control distortion. In what order do you deposit the segments, and how many segments will you make?
  1. Divide the total weld length by the increment length: 12 in. ÷ 3 in. = 4 segments.
  2. Number the segments 1 through 4 from left (start) to right (finish) in the direction of overall weld progression.
  3. Back-step order: begin segment 4 (rightmost), weld leftward for 3 inches; move to segment 3, weld leftward for 3 inches; move to segment 2, weld leftward for 3 inches; finish with segment 1, weld leftward for 3 inches.
  4. Each new segment is started at the end (right side) of the previous segment's starting point, so weld toes blend and shrinkage forces oppose each other.
Answer: Four 3-inch segments deposited in order 4 → 3 → 2 → 1 (right to left within each segment, progressing leftward across the joint). This back-step pattern reduces net longitudinal shrinkage compared with a single continuous weld run.

The math in this lesson1

Learners compute wait time cooling from 510 °F to 450 °F at 15 °F per minute and segment a 12 in back-step weld.
6.RP.3
In class: Weld each joint in the approved sequence, adhering strictly to the WPS essential variables; use a contact or infrared thermometer to verify preheat and interpass temperature before each pass and record the reading in the inspection log — if interpass temperature exceeds the WPS maximum, stop, allow the joint to cool, and re-verify before continuing; instructor conducts random spot-check observations during welding. · After each completed pass, chip slag (SMAW/FCAW), wire-brush, and visually inspect the bead for porosity, cracks, undercut, and overlap before depositing the next pass; mark any discontinuity found in the log with location and disposition (grind and re-weld or accept per D1.1 visual criteria); instructor must be notified of any crack before grinding proceeds. · Monitor and control distortion throughout fabrication by comparing assembly squareness and pipe alignment to the drawing dimensions at defined hold points (after root, after fill, and after cap for each joint); use back-step sequencing, strong-back repositioning, or controlled heat input adjustments as needed; document every corrective action taken in the inspection log.
Materials: SMAW power source and electrode holder; GMAW or FCAW wire-feed machine with appropriate shielding gas (ER70S-6 wire/C25 blend for GMAW, or E71T-1 flux-core wire for FCAW); GTAW power source with high-frequency start, TIG torch, filler rod, and argon shielding gas; electrodes/filler metals per approved WPS (e.g., E7018, ER70S-2 for pipe root); chipping hammer, wire brush, angle grinder; contact or infrared thermometer; weld gauge; in-process inspection log forms; AWS D1.1 current edition (visual acceptance criteria table); ASME Section IX (acceptance criteria reference); full PPE for each process (auto-darkening helmet set to appropriate shade, leather PPE, respiratory protection as needed per SDS for filler metals in use)
Check: Instructor performs a mid-project hold-point inspection at the completion of all root passes (before fill passes begin), reviewing the inspection log for completeness and verifying root pass appearance against AWS D1.1 visual criteria; at project completion, instructor conducts a formal visual weld inspection (VT) of all welds using a weld gauge and measuring tape, scoring against a 25-point rubric covering weld size, profile, undercut, overlap, surface porosity, cracks, and arc strikes — a score of 80% or higher with zero cracks and zero unrepaired undercut exceeding D1.1 limits is required to proceed to documentation and defense.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

AWS D1.1 Certified Welder — procedure compliance and in-process inspection
1. Which action is required if the measured interpass temperature exceeds the maximum listed in the WPS?
AWS SENSE L1-2 — in-process quality and inspection
2. After completing an SMAW fill pass, what is the correct sequence before depositing the next pass?
AWS D1.1 Certified Welder — discontinuity identification and disposition
3. A crack is discovered in a weld pass during in-process inspection. What is the correct immediate action?
NCCER Welding L1-4 — distortion control and fabrication sequence
4. What is the primary purpose of back-step weld sequencing?
ASME Section IX pipe — filler metal selection
5. Which filler metal is commonly specified for a GTAW root pass on carbon steel pipe, and why?
AWS D1.1 Certified Welder — visual acceptance criteria
6. Under AWS D1.1 visual acceptance criteria for statically loaded structures, which undercut condition is rejectable?
AWS SENSE L1-2 — documentation and quality records
7. What information must the in-process inspection log contain for each weld pass, according to the project requirements?
AWS SENSE L1-2 — GTAW process troubleshooting
8. During GTAW, the tungsten electrode becomes contaminated with base metal. What is the correct corrective action before continuing?
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