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BLD-WELD01-08 · Lesson 7 · 11 clock hours · theory

Stainless Steel, Chrome-Moly, and Non-Ferrous Metals in Construction

Objective. The student compares the weldability challenges of austenitic stainless steel (sensitization), chrome-moly (PWHT requirements), and aluminum (oxide layer, hot cracking) and selects correct PPE and ventilation controls for each.

Stainless Steel, Chrome-Moly, and Non-Ferrous Metals in Construction

Not every weld in a building or industrial plant is mild steel. Structural connections may use austenitic stainless steel for corrosion resistance, high-alloy chrome-moly pipe for elevated-temperature service, or aluminum for lightweight architectural and process applications. Each material family brings its own microstructural hazards and protective requirements. Understanding those hazards before you strike an arc is essential for weld quality, code compliance, and your health.

Austenitic Stainless Steel (304 and 316)

Austenitic grades such as 304 and 316 are governed by AWS D1.6, Structural Welding Code—Stainless Steel. Their microstructure is face-centered cubic (FCC) austenite, which gives excellent toughness and corrosion resistance but creates a specific weld-heat problem: sensitization.

Chromium carbides precipitate at grain boundaries when the base metal or heat-affected zone (HAZ) is held in the temperature range of roughly 800 °F to 1500 °F (427 °C to 816 °C) for too long. That precipitation pulls chromium away from the surrounding matrix, leaving a chromium-depleted zone that corrodes readily—a condition called sensitization. The result in service is intergranular corrosion or stress-corrosion cracking. The corrective actions are: use low-carbon grades (304L, 316L), use stabilized grades (321, 347), or water-quench immediately after welding to move through the sensitization range quickly.

Fillers for austenitic SS follow AWS classification ER308/308L for 304, and ER316/316L for 316. Preheat is generally not required, but interpass temperature should be kept below about 350 °F (177 °C) to limit heat input.

Hexavalent Chromium Hazard — OSHA Cr(VI) Rule

Welding on stainless steel generates hexavalent chromium [Cr(VI)] fume, a confirmed human carcinogen. OSHA's Hexavalent Chromium standard (29 CFR 1910.1026 for general industry; 29 CFR 1926.1126 for construction) establishes an action level and a permissible exposure limit (PEL). Employers must use engineering controls first: local exhaust ventilation (LEV) positioned close to the arc, source capture hoods, or down-draft tables. When engineering controls alone cannot achieve compliance, respiratory protection is required—at minimum a half-face air-purifying respirator fitted with P100 particulate filters combined with organic vapor (OV) cartridges. Employers are also required to perform exposure monitoring to determine actual worker exposure and must provide medical surveillance for workers who exceed the action level. Never weld stainless steel in a confined space without both LEV and supplied-air respiratory protection.

Chrome-Moly Steels (4130 and P91)

Chrome-moly (Cr-Mo) alloys contain chromium and molybdenum as principal alloying elements. Common grades include 4130 (a low-alloy steel used in structural and process applications) and the creep-resistant P91 (9Cr-1Mo-V), which is used in power plant and high-temperature piping. Welding of these materials falls under ASME Section IX for pressure applications.

The microstructure of Cr-Mo steels is typically bainitic or tempered martensitic in the base condition. The key weldability challenge is hydrogen-induced cracking (cold cracking) in the HAZ, driven by the hardenable martensitic microstructure. Preheat is mandatory—the required temperature depends on alloy content and section thickness; consult the applicable code or Procedure Qualification Record (PQR). Low-hydrogen filler metals (E8018-B2 for 4130-range alloys; matching 9Cr-1Mo-V filler for P91) and low-hydrogen welding practices (dried electrodes, clean base metal) are essential.

Post-weld heat treatment (PWHT) is required for most Cr-Mo welds. PWHT tempers the as-welded martensite, relieves residual stress, and restores toughness. For P91 specifically, skipping PWHT leaves the HAZ in a hard, brittle, untempered martensitic condition that cannot withstand the creep and thermal-cycling stresses of high-temperature service—a serious integrity risk. PWHT temperature and hold time are specified by the applicable code and the WPS.

Ventilation requirements: Cr-Mo fume contains chromium and other alloy elements; use LEV and appropriate respiratory protection in accordance with OSHA fume standards. Low-hydrogen electrodes must be kept in a heated rod oven and removed only as needed.

Aluminum (6061-T6)

Aluminum alloys are governed by AWS D1.2, Structural Welding Code—Aluminum. The 6061-T6 alloy (Al-Mg-Si, heat-treated to T6 temper) is widely used in architectural, transportation, and light industrial construction.

Aluminum has two primary weldability challenges:

1. Tenacious oxide layer. Aluminum oxide (Al₂O₃) melts at approximately 3700 °F (2040 °C), far above aluminum's melting point of about 1220 °F (660 °C). If the oxide is not removed before and during welding, it creates inclusions and prevents fusion. GTAW uses AC current specifically because the electrode-positive half-cycle provides cathodic cleaning—it breaks up and removes the oxide layer. GMAW (MIG) on aluminum uses DCEP and a push technique with a dedicated clean wire liner and wire brush.

2. Hot cracking (solidification cracking). The wide solidification range of many aluminum alloys leaves a semi-solid weld pool prone to cracking as it shrinks. Using the correct filler metal dramatically reduces this risk: ER4043 (Al-Si) or ER5356 (Al-Mg) are the common choices for 6061. Both are chosen to disrupt the binary alloy solidification range. Porosity in aluminum GMAW is typically caused by hydrogen pickup from moisture (on the base metal, wire, or shielding gas) or inadequate shielding gas coverage (flow rate too low, drafts, or contaminated gas). Corrective actions: clean and dry the base metal and filler wire, verify shielding gas (100% argon) flow rate and nozzle condition, and store filler wire sealed.

PPE note: Aluminum fume is an irritant; use general LEV. Ozone and nitrogen oxides are produced at higher levels with aluminum GTAW—ventilation is critical.

Summary Rule

Match your process, filler, preheat, PWHT, PPE, and ventilation to the base metal. No single set of defaults works across material families. Always consult the applicable code (D1.6, D1.2, or ASME IX), the WPS, and OSHA fume requirements before you weld.

Key terms8

SensitizationPrecipitation of chromium carbides at austenitic stainless steel grain boundaries during exposure to roughly 800–1500 °F, depleting chromium and reducing corrosion resistance in those zones.
Hexavalent Chromium [Cr(VI)]A confirmed human carcinogen produced in fume when welding or cutting stainless steel or other chromium-containing alloys; regulated by OSHA with an established action level and PEL.
Post-Weld Heat Treatment (PWHT)A controlled heating and holding cycle applied after welding to temper hard microstructures, relieve residual stress, and restore toughness; mandatory for most chrome-moly welds under applicable codes.
Untempered MartensiteThe hard, brittle microstructure left in hardenable alloy steel HAZs when PWHT is omitted; associated with low toughness and susceptibility to cracking in service.
Aluminum Oxide LayerA surface film of Al₂O₃ on aluminum that melts at roughly 3700 °F, far above aluminum's melting point; must be removed by cathodic cleaning (AC GTAW) or mechanical cleaning to prevent inclusions and lack of fusion.
Hot Cracking (Solidification Cracking)Cracking that occurs in or near the weld pool as the weld metal solidifies and shrinks; common in aluminum alloys due to their wide solidification range; controlled by proper filler metal selection.
Local Exhaust Ventilation (LEV)An engineering control that captures contaminated air at or near the source (the arc) before it can reach the welder's breathing zone; the preferred first-line control for Cr(VI) and other welding fumes.
Low-Hydrogen PracticeThe combination of dried low-hydrogen electrodes, clean joint preparation, and proper storage conditions used to minimize diffusible hydrogen and prevent hydrogen-induced cracking in hardenable steels.
In class: Complete a comparison table for three material families (austenitic SS 304/316, Cr-Mo 4130/P91, and 6061-T6 aluminum) covering: microstructure, primary weldability hazard, required filler metal family, preheat/PWHT need, and special PPE or ventilation requirement (e.g., stainless fume = Cr-VI—OSHA hexavalent chromium rule). · Review an OSHA hexavalent chromium exposure fact sheet and describe in writing the engineering controls (local exhaust ventilation), respiratory protection (minimum half-face respirator with P100/OV cartridge), and exposure monitoring requirements applicable when welding stainless steel. · Solve three scenario problems: (a) Why did the 304 SS weld crack in the HAZ after slow cooling—explain sensitization and carbide precipitation; (b) A P91 pipe repair was made without PWHT—what microstructural consequence is expected?; (c) An aluminum GMAW bead shows porosity—identify two likely causes and the corrective action for each.
Materials: OSHA Hexavalent Chromium standard fact sheet, AWS D1.6 and D1.2 summary excerpts, comparison table worksheet, scenario problem set, half-face respirator and P100/OV cartridge for demonstration
Check: Comparison table complete and accurate for all three material families; OSHA Cr-VI controls correctly listed; all three scenario problems answered correctly with metallurgical reasoning. 80% minimum.

Practice check8

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

Metallurgy and Weldability
1. A welder fabricates a 304 stainless steel bracket using a slow interpass cooling rate. After the weld cools, corrosion appears along the HAZ. What is the most likely cause?
Metallurgy and Weldability
2. Which corrective action best prevents sensitization when welding standard 304 stainless steel?
Safety and Health / OSHA Compliance
3. OSHA's Hexavalent Chromium standard requires employers to use what control method first when workers weld stainless steel?
Safety and Health / OSHA Compliance
4. When welding stainless steel and engineering controls do not fully reduce Cr(VI) exposure below the PEL, what is the minimum respiratory protection required by OSHA?
Metallurgy and Weldability
5. A P91 pipe repair was completed without the required PWHT. What microstructural consequence is most expected in the HAZ?
Metallurgy and Weldability
6. Why is PWHT required for most chrome-moly (Cr-Mo) steel welds?
Metallurgy and Weldability
7. An aluminum GMAW weld on 6061-T6 shows scattered porosity throughout the bead. Which two causes and corrective actions are most likely?
Process Knowledge and Technique
8. When performing GTAW on 6061-T6 aluminum, alternating current (AC) is preferred over DCEN primarily because AC provides:
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