High Tensile Steel
Steel is probably the most important of all the base metals. It is the most commonly welded metal in maintenance applications. Most steels are welded easily without problems. However, some steels can cause problems when welded unless certain precautions are taken. The steels we will be discussing in this section will be the low alloy grades.Low alloy steel has less than 3{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} of alloying elements. The most common grade of low alloy steels is the AISI (American Iron Steel Institute) grade 1020. The Society of Automotive Engineers (SAE) uses a similar designation system. These two organizations dictate the specifications of all steels. Other types of low-alloy steels are as follows:
AISI-SAE Low Alloy Steel Designations
10XX Plain carbon steel
11XX Resulphurized carbon steel
12XX Rephosphorized and resulphurized steels
13XX Manganese steel
23XX Nickel steel
33XX Nickel Chromium steel
40XX Molybdenum steel
41XX Chromium – Molybdenum steels
43XX Nickel chrome – Molybdenum steels
51XX Chromium steel
61XX Chromium Vanadium steels
86XX Nickel Chromium Molybdenum steels
The first two digits of the designation indicate the type of steel according to its major alloying element. The last two digits of the designation indicate the amount of carbon in the alloy in points (100 points =1 {c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26}). The 1020 classification is commonly called mild steel. The 10XX part of the designation indicates that it is a plain carbon steel; the XX20 part of the designation indicates 20 points of carbon or 1/5 of 1{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26}. This amount of carbon makes the steel strong but is not enough to cause hardness problems. If, however, the steel would have 40 points of carbon, it would be called 1040 steel.
The higher the carbon content in a steel, the higher the hardness will be after welding, unless certain preheating procedures are used. When steels with high carbon contents are welded and cooled rapidly, the weld deposit and heat affected zone will be very hard. Depending on the location of the weld joint it is either restrained or unrestrained. (See Figure 1 following page) If unrestrained, the weld can contract as much as needed. If restrained, that is, still connected to another area, the contraction will cause another crack to develop, causing the weld area to literally pull away from the connected area. By applying certain precautions, cracks in restrained joints can be avoided and successful welds made. The key is to determine the carbon content of the steel being welded. Remember, cracks will come from higher carbon content steels cooling rapidly. If the steel has a low carbon content, it will remain relatively soft or ductile. The shrinkage that will occur won’t create problems because of the ductility of the weld area. An easy way to determine carbon content of the base metal is to use a torch to heat an edge or corner to red heat, then quench in water. Using a hand file, try to file the area that was heated and quenched. If it files easily, the carbon content is low; if it is difficult to file, it has a high enough carbon content to create problems. The more difficult it is to file, the higher the carbon content. When medium and high carbon steel are welded, pre and post-heat procedures can be used to help assure crack-free deposits. Following is a table with the recommended preheat for common grades of low alloy steels as well as some commonly used types of trademark steels in this classification.
