Product design and manufacturing often involves trade-offs to optimize product performance, reliability, and cost. One aspect of cost includes the time and effort required to perform the manufacturing steps. In the case of fabricating carbon steel components, free-machining steels are often used to improve ease of machining. However, if components made of free-machining steel must be welded together, then issues arise, and costs rise, because of the poor weldability of these steels.
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The machinability of carbon steel containing up to about 0.5% carbon is improved by the addition of sulfur. Most resulfurized steels contain 0.08 to 0.13% sulfur, though some alloys allow sulfur content as high as 0.35%. The sulfur combines with manganese to form manganese sulfide (MnS) inclusions within the steel. These inclusions enhance the formation of microvoids during machining, which leads to the formation of broken chips rather than continuous chips. However, steels are typically specified to have a maximum of 0.050% sulfur because steel weldability decreases with increasing sulfur content. So, the number of MnS inclusions is small in standard grades of steel.
Phosphorous and lead may also be added to resulfurized low carbon steel. Phosphorus increases the strength and hardness of the ferrite phase in the steel. This promotes chip breaking rather than the formation of long, stringy chips. Lead is present as soft particles that enhance the formation of microvoids during machining.
Welding Free-Machining Steel
Free-machining steel with sulfur and phosphorous has poor weldability because the MnS inclusions and phosphorous compounds have a lower melting point than the steel. As the weld metal cools and solidifies stresses start to build across the weld due to shrinkage. Due to the presence of the still molten low melting point compounds within the weld metal the metal grains tear apart under the shrinkage stresses, resulting in solidification cracks.
The major concern with lead is its toxicity because lead can melt during welding and volatilize into the weld fumes. Occasionally, lead may cause weld porosity and embrittlement.
Welding free-machining steel is usually inadvisable because of these welding problems. If one of these steels must be welded, special electrodes must be used to try to reduce the sulfur and phosphorus content of the weld metal. Also, low welding currents should be used to minimize base plate dilution. Still, there is no guarantee that these precautions will eliminate the problem. Sometimes the best that can be achieved is a reduced level of cracking.
The amount of cracking that is tolerable will depend on the required service conditions and reliability of the weld. For a weld joint that will be part of a structural member, any amount of cracking is unacceptable. For a weld joint that is part of a structure that will bear light or no loads, some amount of cracking may be acceptable.
Design solutions to prevent weld cracking
Another solution to this problem is to select a standard carbon steel alloy with a composition, microstructure, and hardness that enables optimization between ease of machining and ease of welding, while meeting the product performance and reliability requirements.
Low and medium carbon steel for machining consists of ferrite and spheroidized cementite or ferrite and pearlite. Ferrite is a soft material and cementite is a hard material. Pearlite is a composite consisting of plates of ferrite and cementite. The amount of cementite or pearlite in a steel increases as the steel carbon content increases.
Spheroidized and partially spheroidized cementite in a ferrite matrix.
Ferrite can be readily cut and causes little tool wear. However, it contributes to the formation of a built-up edge on the tool because of its low hardness. The presence of large quantities of massive cementite particles can cause significant wear on a tool since cementite is very hard. Pearlite is harder than ferrite and generally causes greater tool wear, with wear increasing as pearlite plate spacing decreases. However, a built-up edge is less common when machining pearlite than when machining ferrite.
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A few different heat treatments are used to obtain the different microstructures. Normalizing and full annealing result in ferrite and pearlite. Among normalized and annealed steels, those with lower hardness and smaller amounts of pearlite can be machined at higher speeds for equal tool lives. Spheroidization annealing is used to obtain ferrite and spheroidized cementite.
The machinability of as-rolled or annealed low-carbon steel improves with increasing pearlite content and with smaller ferrite grain size because microvoids form at the interface between pearlite and ferrite. Maximum machinability of low-carbon steels is achieved at 0.15 to 0.25% carbon in the as-rolled or annealed condition.
With medium carbon steels, normalizing or annealing, combined with cold drawing give a slight increase in machinability compared to as-rolled cold-drawn steel.
Cold working increases the hardness of ferrite, which results in shorter chip lengths and less built-up edge on the tool. However, using cold-worked steel may require a stress relief heat treatment prior to machining to minimize distortion during machining.