Base Metal Preparation Flashcards

A fabricator is preparing to weld hot-rolled carbon steel that has a bluish-gray, flaky surface. What is this layer called, and what is the primary reason it must be removed before welding?

Answer: Mill scale; it can introduce impurities and cause weld defects like porosity.

The bluish-gray, flaky surface on hot-rolled steel is called mill scale, which is a form of iron oxide. It must be removed before welding because it can break down in the heat of the arc, releasing oxygen and other impurities into the molten weld pool. This contamination can lead to significant weld defects, such as porosity and poor fusion, which weaken the final weldment.

When preparing a joint on a 2-inch thick, high-carbon steel plate for a critical structural application, which of the following steps is most crucial for preventing hydrogen-induced cracking?

Answer: Applying a preheat of 250-400°F (121-204°C) before welding.

Preheating is a critical step for thick sections of high-carbon steel. It slows the cooling rate of the weld and the heat-affected zone (HAZ), which allows hydrogen that may be present to diffuse out of the material instead of getting trapped and causing cracks. A slower cool-down also produces a more ductile microstructure that is less susceptible to cracking. The AWS D1.1 code provides specific preheat requirements based on material thickness, grade, and hydrogen levels.

A welder is tasked with preparing an aluminum base metal for a high-quality TIG weld. Which of the following preparation sequences is correct?

Answer: Degrease with a suitable solvent, then brush with a dedicated stainless steel wire brush.

The correct procedure for preparing aluminum is to first clean off oils and hydrocarbons with a solvent, and then mechanically remove the tough outer layer of aluminum oxide. A dedicated stainless steel brush should be used after degreasing to avoid embedding contaminants into the soft aluminum surface. Using a carbon steel brush or a contaminated grinding disc would introduce ferrous particles, leading to contamination and weld defects.

Which of the following contaminants on a base metal is most likely to cause weld porosity if not properly removed?

Answer: Moisture, oil, or grease

Moisture, oils, grease, and similar hydrocarbon-based contaminants are a primary cause of weld porosity. When subjected to the high heat of the welding arc, these substances vaporize and release gases (such as hydrogen and carbon dioxide) that can become trapped in the solidifying weld metal, creating gas pockets or pores.

In preparing a butt joint for a complete joint penetration (CJP) weld on a 1-inch thick plate, a welder grinds the edges to form a groove. What is the primary purpose of creating this weld groove?

Answer: To increase the travel speed of the welding process.

Creating a groove, such as a V-groove or U-groove, is essential for achieving complete joint penetration on thicker materials. The groove allows the welder to access the root of the joint and ensure that the weld fuses completely through the entire cross-section of the base metal, creating a strong, continuous joint.

When using a grinder to prepare a bevel on a steel plate, which of the following is a critical safety and quality practice?

Answer: Ensuring the prepared surface is smooth and free of deep gouges or grinding marks.

A properly prepared surface should be smooth and uniform. Deep grinding marks or gouges can act as stress risers, becoming potential initiation points for cracks in the final weld. They can also trap contaminants. While speed is important, quality of the surface finish is paramount for a sound weld. Safety regulations strictly prohibit removing guards and using underrated wheels.