| Carbon steels are supplied in the as-rolled, normalised, or hardened and tempered condition, with the best properties developed by hardening and tempering. The effect of carbon content on the tensile strength, elongation to failure and hardness of annealed plain carbon steel is shown in figure 1. | 
| | Figure 1. The effect of carbon content and heat treatment on the typical properties of plain carbon steels. | Very Low Carbon Content Steels Very low carbon content (up to 0.05%C). These steels are ductile and have properties similar to iron itself. They cannot be modified by heat treatment. They are cheap, but engineering applications are restricted to non-critical components and general panelling and fabrication work. Low Carbon Content Steels Low carbon content (0.05% to 0.2%C) e.g. 080M15, 150M19, 220M07, AISI 1006, AISI 1009, AISI 1020. These steels cannot be effectively heat treated, consequently there are usually no problems associated with heat affected zones in welding. Batches which are free of 'tramp' elements such as chromium are ductile with good forming properties, as little work hardening is exhibited. However, chromium as low as 0.1% and vanadium and molybdenum contents as low as 0.05% can have a dramatic effect on hardenability. Surface properties can be enhanced by carburising and then heat treating the carbon rich surface. High ductility results in poor machinability, although these steels can be machined if high spindle speeds are employed. More commonly sulphur and lead are added to form free machining inclusions. Low quality steels with high quantities of sulphur and phosphorus will have better machinability than good quality steels which are clean and free from oxides and slag inclusions. This group represents the bulk of the market for general purpose steel, finding usage in car bodies, ships and domestic appliances. Stainless steels and aluminium alloys compete with these steels in certain areas. Medium Carbon Content Steels Medium carbon content (0.2% to 0.5%C) e.g. 070M20, 080M40, 216M44, AISI 1023, AISI 1030, AISI 1046. Heat treatment and work hardening are now effective methods for modifying mechanical properties. Hardenability increases in proportion to carbon content. Welders must now take note of the hardening effects in the heat affected zone and take precautions against excessive energy input, as increased hardenability results in an increased likelihood of brittle structures forming. All common alloying elements increase the hardenability and hence .a 'carbon equivalent' scale has been devised as an approximate guide to weldability (figure 2). | Carbon Equiv (CE%) = C% + (Mn%/6) + ((Cr%+Mo%+V%)/5) + ((Ni%+Cu%)/15) | For | CE%<0.14 | Excellent weldability, no special precautions necessary | | | 0.14<CE%<0.45 | Martensite is more likely to form, and modest preheats with low hydrogen electrodes become necessary | | | CE%>0.45 | Extreme complications, weld cracking is very likely, hence preheat in the range 100-400°C and low hydrogen electrodes are required | | | Figure 2. Method for calculating the carbon equivalent for alloyed steels. |
In the normalised condition, machinability is improved compared with low carbon steels due to their lower ductility and it can be further enhanced with the addition of sulphur or lead if special 'free machining' properties are required. Ductility and impact resistance is, however, reduced. The corrosion resistance of these steels is similar to low carbon steel, although small additions of copper can lead to significant improvements when weathering performance is important. Most steels in this category contain some silicon and manganese, which are added as deoxidising and desulphurising elements during manufacture. While the quantities present are not considered to effect mechanical properties, an indication of the quality of the steel is given by the phosphorus and sulphur content, where the lower the content, the higher the quality. This category represents medium strength steels which are still cheap and command mass market. They are general purpose but can be specified for use in stressed applications such as gears, pylons and pipelines. Medium-High Carbon Content Steels Medium-high carbon content (0.5% to 0.8%C) e.g. 070M55, 0S0M50, AISI 1055, AISI 1070. These steels are highly susceptible to thermal treatments and work hardening. They easily flame harden and can be treated and worked to yield high tensile strengths provided that low ductility can be tolerated. For example, spring wire in this category can have an ultimate tensile strength (UTS) >2GPa. Clearly, welders must take care to prevent heat affected zone (HAZ) cracking with these steels, and specialist advice should always be obtained. The carbon equivalent can be used to evaluate potential welding problems. Although high strengths and hardnesses are attainable, impact strengths are poor. These steels are not normally used in stressed applications subjected to shock. They are used where hardness is valued, such as for blades, springs, collars, etc. High Carbon Content Steels High carbon content (>0.8%C) e.g. 050A86, 080A86, AISI 1086, BS 1407. Cold working is not possible with any of these steels, as they fracture at very low elongation. They are highly sensitive to thermal treatments. Machinability is good, although their hardness requires machining in the normalised condition. Welding is not recommended and these steels must not be subjected to impact loading. These steels can have UTSs greater than 1 GPa, and care needs to be taken to avoid hydrogen embrittlement following electroplating. Advice should be sought from the plating shop. As with the medium-high plain carbon steels, steel with >0.8%C is used for components requiring high hardness such as cutting tools, blades, etc. |