🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Welding for Auto Body & Collision Repair › Lesson 6
TRN-AUTO25-04 · Lesson 6 · 16 clock hours · demo

Welding High-Strength and Ultra-High-Strength Steel

Objective. Students will state the process and heat-input restrictions for welding HSS and UHSS and correctly set up a MIG welder for a DP590 coupon following OEM reference guidance.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why HSS and UHSS Demand Special Welding Rules

Modern vehicles use High-Strength Steel (HSS) and Ultra-High-Strength Steel (UHSS) in structural components—door pillars, rocker panels, roof rails, and front/rear rails—to save weight while protecting occupants. These steels earn their strength through precise metallurgical processes: controlled rolling, alloying, and in many cases a heat-treatment called martensitic transformation. The very properties that make them strong also make them vulnerable to improper welding.

When you apply excessive heat to HSS or UHSS, you alter the grain structure inside the heat-affected zone (HAZ)—the metal immediately surrounding the weld that never actually melted but was exposed to elevated temperatures. In mild steel this is a minor concern. In UHSS such as dual-phase 590 (DP590), the HAZ can lose a significant portion of its tensile strength, turning a safety-critical zone into a weak spot that will not perform as the OEM designed.

The core rule: Always follow the vehicle manufacturer's (OEM) published repair procedure. If the OEM says no heat, no heat. If the OEM says section only at designated locations, those locations are not suggestions.

Understanding the Steel Grades

Mild steel (typically under 270 MPa yield strength) is forgiving. You can weld it with a wide range of heat inputs and it will retain acceptable properties.

High-Strength Steel (HSS) covers steels roughly in the 270–550 MPa yield-strength range. Common examples include high-strength low-alloy (HSLA) steels. These tolerate limited heat input but begin to show HAZ softening if you linger too long or run amperage too high.

Ultra-High-Strength Steel (UHSS) covers steels above approximately 550 MPa, including dual-phase (DP), complex-phase (CP), martensitic (MS), and press-hardened steels (PHS/boron steel). DP590 is a dual-phase steel with a minimum tensile strength around 590 MPa. Martensitic steels can exceed 1,500 MPa. Once you overheat UHSS, the martensite in the HAZ reverts to softer ferrite and the part can no longer protect the occupant in a crash.

Heat-Input Restrictions

Heat input in MIG welding is influenced by voltage, wire-feed speed (which drives amperage), and travel speed. The practical rules for HSS/UHSS are:

Filler Wire Selection

For MIG welding of HSS and most UHSS structural repair, ER70S-6 solid wire is widely specified. It provides adequate tensile strength for the weld deposit and is compatible with short-circuit transfer at the low-heat settings required. Always verify the OEM procedure; some high-strength or boron-steel components may have specific wire or process requirements. Never substitute wire grades without consulting the procedure.

Setting Up the MIG Welder for DP590 (Plug Weld)

1. Confirm settings from the OEM chart or I-CAR reference for 18-gauge DP590. Typical starting parameters for short-circuit MIG on thin HSS are in the low-voltage, moderate wire-feed range—consult your specific machine's chart and the reference material provided.

2. Set gas: 75% Argon / 25% CO₂ (C25) is a common shielding gas for short-circuit MIG on steel. Verify with your procedure.

3. Drill plug-weld holes in the top coupon per the specification (commonly 8 mm diameter for thin steel).

4. Clamp coupons tightly together. Gaps increase heat needed to bridge the joint, which increases HAZ degradation.

5. Weld in a circular motion starting at the edge of the hole and filling to the center. Do not pause in one spot.

6. Check interpass temperature before adding the next weld. Allow cooling as specified.

Quality Check: Peel-Back Test

The peel-back (pull) test is the industry-standard destructive test for plug welds. The weld passes if failure occurs in the base metal around the weld nugget, not at the weld interface itself. A weld that pulls cleanly out of the hole (interface failure) is undersize or cold. Your instructor will perform this test live so you can observe the difference.

Safety

Key terms10

HSS (High-Strength Steel)Steel with yield strength roughly in the 270–550 MPa range, used in structural body components; requires controlled heat input during welding to avoid HAZ softening.
UHSS (Ultra-High-Strength Steel)Steel with yield strength above approximately 550 MPa, including dual-phase, martensitic, and press-hardened grades; highly sensitive to heat input because elevated temperatures destroy the microstructure that provides its strength.
DP590 (Dual-Phase 590)A specific UHSS grade with a minimum tensile strength around 590 MPa; composed of a ferrite matrix with martensite islands; commonly used in structural rails and pillars.
Heat-Affected Zone (HAZ)The region of base metal adjacent to a weld that was not melted but was heated enough to alter its grain structure and mechanical properties.
Short-Circuit MIG (GMAW-S)A MIG welding transfer mode in which the wire contacts the puddle repeatedly, producing a low-heat, low-spatter arc; preferred for thin HSS/UHSS to limit heat input.
Interpass TemperatureThe temperature of the base metal immediately before a subsequent weld pass is started; OEM procedures often specify a maximum interpass temperature for HSS/UHSS to prevent cumulative heat damage.
ER70S-6A commonly specified solid MIG filler wire with a minimum tensile strength of approximately 70,000 psi (70 ksi); the standard wire for structural MIG welding of HSS in collision repair.
Peel-Back (Pull) TestA destructive quality test for plug welds in which the joint is mechanically forced apart; a passing result shows base-metal tearing around the weld nugget rather than interface separation.
Sectioning LocationAn OEM-designated area on a structural component where a repair cut and weld joint is permitted; welding outside these locations can compromise crash-structure integrity.
Tempilstik (Temperature-Indicating Stick)A crayon-like device that melts at a specific calibrated temperature; used to monitor interpass temperature on HSS/UHSS welds without requiring electronic instruments.

Worked examples4

1. You are measuring HAZ width on two DP590 coupons after the instructor demo. Coupon A (correct settings) shows a HAZ of 3 mm on one side of the weld. Coupon B (excessive heat) shows a HAZ of 9 mm on the same side. How much wider is the HAZ on Coupon B compared to Coupon A, expressed as a difference and as a ratio?
  1. Step 1 – Find the difference: 9 mm − 3 mm = 6 mm wider on Coupon B.
  2. Step 2 – Find the ratio: 9 mm ÷ 3 mm = 3. Coupon B's HAZ is 3 times as wide as Coupon A's.
Answer: Coupon B's HAZ is 6 mm wider than Coupon A's, and it is 3 times as wide. This illustrates how excess heat dramatically expands the zone of potential strength loss in UHSS.
2. Your OEM procedure states the interpass temperature limit is 150 °F. You check the coupon with an infrared thermometer after the first plug weld and read 210 °F. How many degrees must the coupon cool before you weld the next plug?
  1. Step 1 – Identify current temperature: 210 °F.
  2. Step 2 – Identify maximum allowed temperature: 150 °F.
  3. Step 3 – Calculate required cooling: 210 °F − 150 °F = 60 °F of cooling required.
Answer: The coupon must cool at least 60 °F—from 210 °F down to 150 °F or below—before the next weld pass.
3. You are drilling plug-weld holes in an 18-gauge DP590 top coupon. The procedure calls for holes spaced 40 mm apart, center to center, along a 200 mm joint. Starting from one end at 20 mm from the edge, how many holes do you drill, and where is the center of the last hole?
  1. Step 1 – First hole center: 20 mm from the starting edge.
  2. Step 2 – Each subsequent hole adds 40 mm. Hole 2: 20 + 40 = 60 mm. Hole 3: 60 + 40 = 100 mm. Hole 4: 100 + 40 = 140 mm. Hole 5: 140 + 40 = 180 mm.
  3. Step 3 – Check if a 6th hole fits: 180 + 40 = 220 mm, which exceeds 200 mm, so stop at hole 5.
  4. Step 4 – The last hole center is at 180 mm from the starting edge.
Answer: You drill 5 holes. The last hole center is 180 mm from the starting edge.
4. A tempilstik rated at 150 °F is placed on the coupon. After welding, the mark melts completely. A second tempilstik rated at 200 °F does not melt. What does this tell you about the coupon's peak interpass temperature, and did it stay within a 150 °F limit?
  1. Step 1 – Interpret the 150 °F stick: it melted, so the coupon reached at least 150 °F.
  2. Step 2 – Interpret the 200 °F stick: it did not melt, so the coupon stayed below 200 °F.
  3. Step 3 – Conclude: the peak temperature was somewhere between 150 °F and 200 °F.
  4. Step 4 – Compare to limit: the coupon reached at least 150 °F, which meets the maximum limit exactly but does not confirm it stayed below 150 °F—it confirms the surface hit 150 °F or slightly above.
Answer: The coupon's peak temperature was at or just above 150 °F but below 200 °F. Because the 150 °F stick melted, the coupon at least reached the limit; in practice this is a marginal result and the technician should allow more cooling time before the next pass to ensure the limit is not exceeded.

The math in this lesson1

Learners compare HAZ widths of 3 mm and 9 mm as a 6 mm difference and a 3-to-1 ratio.
6.RP.1
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor-led comparison demo: weld DP590 coupon with correct settings, then weld a second coupon with excessive heat; students measure distortion and inspect heat-affected zone (HAZ) width with a ruler to observe difference in degradation. · Students review an OEM repair procedure for a structural rail section made from UHSS and write a one-paragraph repair plan identifying: process permitted, heat restrictions, sectioning location, and filler wire specification. · Students set up the MIG welder for a DP590 coupon using the settings chart and weld two plug welds; instructor performs a live peel-back test in front of the class and the group evaluates the result against the quality standard.
Materials: MIG welders, DP590 coupons (18 ga), ER70S-6 wire, OEM structural repair procedure printout, ruler and temperature-indicating sticks (tempilstik) or infrared thermometer, I-CAR Steel Fundamentals reference, lab log forms
Check: Students verbally answer three instructor questions about HSS/UHSS heat restrictions with 100% accuracy; plug-weld peel-back test on DP590 coupon shows base-metal failure to pass the demo—no written exam, but lab log entry required.

Practice check8

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

I-CAR Steel Fundamentals / Structural Repair
1. Why does welding UHSS with excessive heat input reduce the strength of the heat-affected zone?
ASE B2 / Welding and Cutting
2. Which MIG transfer mode is preferred when welding thin HSS and UHSS structural panels to minimize heat input?
I-CAR Structural Repair / OEM Procedures
3. A technician needs to repair a UHSS front rail. The OEM procedure lists two approved sectioning locations. The technician determines it would be faster to section 50 mm forward of the designated location. What is the correct action?
ASE B2 / Welding Quality and Testing
4. A plug-weld peel-back test is performed on a DP590 coupon. The weld pulls cleanly out of the hole with a smooth, button-shaped disc of weld deposit and no base metal attached. What does this result indicate?
I-CAR Steel Fundamentals / Welding Consumables
5. Which filler wire is most commonly specified for short-circuit MIG welding of HSS structural components in collision repair?
I-CAR Structural Repair / OEM Procedures
6. Before applying heat to a UHSS structural component, what is the first resource a technician should consult?
ASE B2 / Welding Procedures and Heat Control
7. A technician is welding plug welds on an 18-gauge DP590 coupon. After the first weld, the infrared thermometer reads 240 °F and the OEM procedure states a 150 °F interpass temperature limit. What should the technician do?
ASE B2 / Safety and Hazardous Materials
8. Which personal protective equipment (PPE) item is specifically important when welding zinc-coated HSS panels, in addition to a welding helmet and gloves?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →