Welding Metallurgy
Metallurgy refers to the chemistry of metals and welding metallurgy looks at the influence of welding heat on the properties of metals. The intense, high heat of the welding process can have a dramatic effect on the mechanical and chemical properties of the weldment. Understanding the materials you'll be welding on and how to generate desired weld metal properties is key to successful welding, fabrication and repair of metals.
Carbon Steels
Carbon steels are very commonly used for various applications. These steels are classified according to their carbon content or % by weight which determines the hardenability of the steel and in turn, the weldability of the steel. Steels are classified into the following categories:
1. Low carbon steels: These steels contain lower amounts of carbon, which reduce hardenability and provide good weldability. Low carbon steels may also include MILD steels and these classifications have a carbon content below 0.30%.
2. Medium carbon steels: These steels contain between 0.30% - 0.50% carbon. This additional carbon increases mechanical properties like strength and hardness, but it decreases weldability because pre and post weld heat treatment must be used to avoid creating an embrittled weld and heat affected zone.
3. High carbon steels: These steels contain greater than 0.50% carbon and they are extremely hardenable. Therefore, weldability is low as extreme pre and post weld heat treatment must be used to avoid creating an embrittled weld and heat affected zone.
1. Low carbon steels: These steels contain lower amounts of carbon, which reduce hardenability and provide good weldability. Low carbon steels may also include MILD steels and these classifications have a carbon content below 0.30%.
2. Medium carbon steels: These steels contain between 0.30% - 0.50% carbon. This additional carbon increases mechanical properties like strength and hardness, but it decreases weldability because pre and post weld heat treatment must be used to avoid creating an embrittled weld and heat affected zone.
3. High carbon steels: These steels contain greater than 0.50% carbon and they are extremely hardenable. Therefore, weldability is low as extreme pre and post weld heat treatment must be used to avoid creating an embrittled weld and heat affected zone.
Stainless Steels
Stainless steels are classified as ferrous or steel-based alloys with a minimum of about 10.5% chromium added to improve corrosion resistance. Stainless steels may also have other elements like nickel or molybdenum that further improve mechanical or chemical properties over one another and certainly over plain carbon steels.
Stainless steels are classified into three (3) main categories based on alloy content and the resulting microstructure of the alloy. Each has varying levels of weldability and unique properties that must be accounted for in the welding process.
1. Austenitic: This grade of stainless is represented by the 2xx or 3xx designation and they are often used for food-grade applications.
2. Ferritic: This grade of stainless is represented by some of the 4xx designated alloys and they typically lack the corrosion resistance of austenitic grades. However, they have good toughness - especially at elevated temperatures - which makes them ideal for applications like automotive exhausts.
3. Martensitic: This grade makes up the balance of 4xx series alloys. These alloys have a hardened microstructure which leads to poor weldability with respect to the other grades. The mechanical properties of these grades make them ideal for applications such as cutlery because they hold an edge so well.
Stainless steels are classified into three (3) main categories based on alloy content and the resulting microstructure of the alloy. Each has varying levels of weldability and unique properties that must be accounted for in the welding process.
1. Austenitic: This grade of stainless is represented by the 2xx or 3xx designation and they are often used for food-grade applications.
2. Ferritic: This grade of stainless is represented by some of the 4xx designated alloys and they typically lack the corrosion resistance of austenitic grades. However, they have good toughness - especially at elevated temperatures - which makes them ideal for applications like automotive exhausts.
3. Martensitic: This grade makes up the balance of 4xx series alloys. These alloys have a hardened microstructure which leads to poor weldability with respect to the other grades. The mechanical properties of these grades make them ideal for applications such as cutlery because they hold an edge so well.
Aluminum
Aluminum alloys are often the most mysterious metals to weld. Designations for these alloys are based on the major alloying element, which defines their properties and often defines weldability of the alloy. Aluminum forms a high-melting surface oxide that must be removed prior to welding - which is often a barrier to achieving good weld quality. Some aluminum alloys are heat treated to improve mechanical properties and others must be work hardened as they do not respond to a heat treatment. Depending on the alloy or grade of aluminum, it may have excellent, good, fair or very poor weldability - with some alloys being classified by the AWS as "un-weldable." In addition, aluminum is prone to solidification cracking or "hot-short" cracking so particular attention must be paid when selecting a filler material.
Alloys are classified by the following designations:
1xxx: commercially pure aluminum, good weldability, non-heat treatable
2xxx: Copper, considered un-weldable, heat treatable
3xxx: Manganese, good weldability, non-heat treatable
4xxx: Silicon, good weldability - although most commonly found as filler metal, non-heat treatable
5xxx: Magnesium, good weldability, non-heat treatable
6xxx: Silicon & Magnesium, good weldability, heat treatable
7xxx: Zinc, considerd un-weldable, heat treatable
Alloys are classified by the following designations:
1xxx: commercially pure aluminum, good weldability, non-heat treatable
2xxx: Copper, considered un-weldable, heat treatable
3xxx: Manganese, good weldability, non-heat treatable
4xxx: Silicon, good weldability - although most commonly found as filler metal, non-heat treatable
5xxx: Magnesium, good weldability, non-heat treatable
6xxx: Silicon & Magnesium, good weldability, heat treatable
7xxx: Zinc, considerd un-weldable, heat treatable