1. There cannot be strengthening beyond about 100000 psi without significant loss in toughness (impact resistance) and ductility.
2. Large sections cannot be made with a martensite structure throughout, and thus are not deep hardenable.
3. Rapid quench rates are necessary for full hardening in medium-carbon leads to shape distortion and cracking of heat-treated steels.
4. Plain-carbon steels have poor impact resistance at low temperatures.
5. Plain-carbon steels have poor corrosion resistance for engineering problems.
6. Plain-carbon steel oxidises readily at elevated temperatures
Phosphorus is an element, which affects primarily the ductility and the toughness of steel and this mostly when the steel is in the quenched and tempered conditions. In fact, the phosphorus has a tendency to react with the iron to form a compound known as iron phosphide (Fe3P) which has the particularity of being brittle. Hence, phosphorus renders steel less tough and ductile while it increases brittleness.
Although the fact, that silicon is not that harmful to steel, still it has some bad effects on its properties. In fact, silicon has the particularity of impairing hot and cold workability and machinability. The presence of Silicon in low carbon steel is also detrimental since it affects the surface quality of the steel.
Oxygen is really a poison to steel. Indeed, when present in steel, it has a very bad effect on its mechanical properties. To be more precise, oxygen reduces the impact strength of steel, whereas it has the tendency to increase its ageing brittleness, red shortness, woody and slanty fractures. In brief Oxygen reduces the toughness of steel.
Like Oxygen, Hydrogen also is injurious to steel as it causes embrittlement by decreasing of elongation and reduction of area without any increase of yield point and tensile strength. Indeed, hydrogen is the source of redoubtable snowflake formation and it favors the formation of ghost lines in the steel structure. Furthermore, atomic hydrogen engendered by pickling penetrates into the steel and forms blowholes. This element also acts as a decarburising agent when it is in the moisted form (at high temperatures).
Sulphur is a trace element, which has a great tendency to segregate (that is to isolate itself in the structure). It also reacts with iron to form iron sulphide which produces red or hot-shortness, since the low melting eutectic forms a network around the grains so that these hold but loosely together, and the grain boundaries may easily break up during hot forming. Sulphur plays a great role also in the drop in weldability, impact toughness and ductility of steel.
Tin is also considered as being a residual element and this simply because, just as steel, it causes hot shortness. In addition to this, tin is also a source of temper embrittlement.
Arsenic, for its part, plays an important role in the rise of temper embrittlement in the properties of steel. Furthermore, it causes a considerable drop in toughness and it also impairs weldability.
This has as effect similar to Arsenic which also cause temper embrittlement and it affects quite considerably the toughness and the ductility of steel.
This is not the most harmful trace element since it only causes a decrease in toughness of the steel.