Lead in Steels


Lead in Steels

 Lead (Pb) (atomic number 82 and atomic weight 207.21) has density of 11.34 gm/cc. Melting point of Pb is 327.5 deg C and boiling point is 1749 deg C. The phase diagram of the Fe-Pb binary system is at Fig 1. Pb has a face centered cubic (f.c.c.) crystal structure.

Fe-Pb phase diagram

 Fig 1 Fe-Pb binary system

Pb is only slightly soluble in liquid or solid steel. When added , Pb does not remain in the metal but it remains separate from and mechanically dispersed in the steel  as submicroscopic metallic inclusions when it solidifies. It does not change the mechanical properties of the steel appreciably.

The sole function of Pb is to improve the machinability of steel. This accounts for the wide use of Pb for high strength steel for which machinability is required. Also, Pb only slightly influences mechanical properties of steel. However, with the growing consciousness of environmental issues in recent years, there is a tendency toward eliminating Pb because it is seen to be one of the pollutants of the environment.

Addition agents and addition practice

Elemental Pb is available as cored wire (Pb micro shots), pellets and fine shots suitable for addition in the continuous casting tundish, ladle metallurgical furnace, or degassing furnace. Alloys containing bismuth (bi) are also available for addition.

Addition of Pb in liquid steel is carefully controlled to ensure its even dispersion in the solidifying steel. Segregation of Pb can be an issue. Pb segregation in steel  can lead to problems during cold working and machining of steel. Due to this Pb bearing steels are normally checked for segregation. The steel sample is usually heated at 700 deg C and is examined for uniform distribution of Pb. This test is known as sweat test.

Pb does not has a high vapour pressure since its boiling point is above the normal temperatures for steel tapping. Still the addition of Pb is accompanied by generation of considerable amounts of fumes. Pb vapours are toxic and must be vented to keep atmospheric concentrations below the permissible thresh hold value limit of 50 micro grams per cubic meter of air. Residual blood Pb levels of the employees working in the area are required to be checked periodically to keep the levels within the recommended maximum value of 40 micro grams per 100 grams.

Rolling and hot working

When Pb is present in steel in concentrations sufficient for the steel to have free machining properties (0.15 % to 0.35 %), there is no detrimental effect of Pb on hot working of steel. Presence of Pb in higher amounts can cause hot shortness but this problem can be readily detected beforehand by the sweat test.

Influence of lead on steels

Pb can be added to both carbon (C) and alloy steels to improve the machinability. The addition of Pb augments the effect of the sulphur (S), reducing friction and permitting increased machine speed. Because of the fine homogenous distribution of Pb, formation of shorter chips and clean faces of cut is achieved which improves the machinability. During machining, Pb melts locally at the tool / work piece interface and acts as a lubricant for reducing the tool wear. The addition of Pb improves the economics of  machining and improves the surface finish of  complex parts made from steel.

While Pb is an element that is added to the liquid steel, it does not remain in solution in the steel since the solubility of Pb in steel is very limited. It remains  in a mechanically dispersed form in the liquid steel  and forms inclusions when the steel solidifies. These inclusions are usually associated with the manganese sulphide inclusions. It is the dark material on the ends of the manganese sulfides as shown in Fig 2.

Lead inclusions in free machining steels Fig 2 Pb inclusions in a sample of free machining steel

Pb has no apparent effect on the yield strength, tensile strength, reduction of area, elongation, and impact strength of steel. In creep resisting alloy steels very small amounts of Pb can be harmful.

Pb does cause some changes in mechanical properties, but they are slight and usually considered tolerable. Due to the presence of Pb, transverse ductility can be reduced in annealed steels. Also high temperature (345 deg C to 425 deg C) impact strength is impaired. Fatigue life suffers as the tensile strength of the steel increases.

Pb promotes formation of large smooth surface spangles when batch hot dip method is used for galvanizing of steel sheets. The Pb level of galvanizing bath to promote spangle formation on steel had been around 0.75 % to 1.25 %. If the amount of Pb present in galvanizing bath exceeds this amount, it settles to the bottom of the galvanizing bath as an immiscible Pb rich liquid with respect to zinc (Zn).

The first galvanize coatings contained as much as 1 % of Pb. During the past 40 years, the presence of such high Pb levels has not been common in the galvanized coatings on steel sheet. Typical concentrations of Pb, where it was intentionally used, in most galvanized sheet made during this time has been less than 0.15 %, often as low as 0.03 % to 0.05 %. Even this lower amount of lead is still sufficient to develop dendritic growth behaviour during the solidification process. Now a days, a typical level of Pb in the galvanizing bath on lines where the Zn coated product requires a well developed spangle pattern is in the range of 0.05 to 0.10 % Pb.

 Heat treatment 

Pb has no intrinsic influence on heat treatment reactions of steel. Pb bearing steels form a slightly more adherent scale. The insulating property of this scale can reduce cooling rates during quenching. Hence Pb bearing steels need more severe quenching for equivalent depth of hardening. This may be the reason why Pb bearing steels are more susceptible to quench cracking.

Applications

Pb is only added to steels for improving machinability. Any steel may be leaded, and Pb free machining versions of many carbon and alloy steels are also widely available. A number of proprietary free cutting Pb bearing steels are also marketed under trade names. Usually, Pb content is held to about 0.2 %, but it may be augmented by the addition of other free machining agents such as Bismuth (Bi).

The positive effect of Pb on machinability of steels is widely utilized property. Pb improves the machinability of high speed steels at low and moderate cutting speeds for tools. Alloying with medium carbon steel with Pb improves the cutting speeds around 30 % and when the cutting speeds remains the same, it improves the tool life by 3 to 5 times. Together with longer tool life and better shape of shavings, the cutting force decreases and the surface quality of work piece increases.

Pb is added to steels used to produce precision parts in watches for example. There are many products with small mechanical parts where the demand is high on their durability. This includes everything from ABS (anti lock braking system) brakes, dental tools, probes, and instrument panels etc. In addition to weathering normal wear and tear, these small mechanical parts are to be light as well as easy to cut, turn, and mill to the desired shape. Traditionally steels used for such parts are normally having Pb in it.