Free Cutting Steels
Free Cutting Steels
Machining is a term which covers a large collection of production processes designed to remove unwanted material from a work-piece. Machining is used to convert cast, forged, or rolled steel into desired shapes, with size and finish specified to fulfil design needs. Almost every produced product has components which need machining, frequently to high precision. Machining processes are frequently the most expensive. Although the metal cutting process still resists theoretical analysis because of its complexity, the application of this process is wide-spread. Machining processes are performed on a wide variety of machine tools (lathes, drill presses, milling machines, and saws etc.).
The majority of industrial applications of machining are in metals. Quite frequently, a metal is selected for a particular application chiefly because it machines well. Cast iron, for example, is known to be machined easily. Other metals, such as stainless steel or titanium, are more difficult to machine. They frequently have high cutting forces, which can result in short cutting tool lives or poor surface finishes. The economic importance of the machining operations in the final cost of a steel part (around 50 % of the value of a machined automotive part is because of the machining costs), has driven an important part of metallurgical studies in order to improve the steel machining performance which is known as machinability.
Machinability is a term used for the totality of material properties, which influence the machining process. It is a general description of the difficulties occurring during the cutting the material of a work-piece. Normally, for the description of machinability, the four main evaluation criteria which are used are (i) process forces, (ii) surface quality, (iii) chip form, and (iv) tool life. Depending on the specific machining task the importance of every single criterion can vary. However, in some cases these four criteria do not describe the machinability of the material of the work-piece sufficiently and hence other criteria like friction coefficient, cutting temperatures, layer formation, rim zone properties, or build up edge (BUE) formation have to be taken into account. However, these criteria are closely linked to the four main evaluation criteria and hence they are normally not considered as separate machinability measures. Further, specified environmental aspects is also an evaluation criterion.
Machinability is the ability of a material to be shaped by removing chips by a cutting tool. The economic importance of the machining operations, within the total cost break-down of the forming process for steel components is high. However, it is very difficult to clearly define a concept of machinability in a way which suits everyone. Hence, in some cases, it is important to improve machinability within the basis of (i) general improvement of machining conditions so as to increase the cutting parameters (speed, feed rate, as depth of cut), for which it is necessary to develop steel grades with lower hardness to reduce cutting forces, and with higher content of low melting point particles, for improving the lubrication effect at the tool / chip interface, (ii) reduction of tool costs, not only by the work performed in the tool itself (composition and geometry), but also reducing the abrasive effect of the steel, and (iii) automation of production processes, and a reduction of man-power effort by a good reproductivity of results among different heats of same steel material.
The vision of the machinability is the capacity of a steel to be machined rapidly (in terms of productivity) by keeping satisfactory tool life, and by avoiding any machining interruption for chips evacuation. This objective is reached economically by using the cheapest routes, such as increasing the sulphur (S) content of the steel and alloying the steel with lead (Pb) intended to be machined. These two routes have been used for decades, and their cumulative effects have been the heart of development of the low carbon free-cutting steels.
Low carbon free cutting steels are mainly applied for serious or mass production parts, such as in the automobile industry, for the production of small components for kitchen machines, or for the production screws, nuts and several kinds of fittings. The steel material is frequently being machined on auto lathes, on multi spindle machines, or on specific application-oriented machine tools. The number of parts produced with one set of tools is quite high. However, it has to be taken into account that a variety of machining processes such as turning, grooving, drilling, tapping, and parting are used. Hence, the steel material has to assure good machinability at a broad range of cutting parameters.
Automated production is important as cycle times are normally a few seconds and the machine tools run all over the week. Operators care for several machine tools and do not handle only one lathe. Hence, special demands are made on the machinability of low carbon free cutting steels. It is obvious that tool life is a main criterion for machinability but at least the same importance arises for the chip form. Good chip breakage is a pre-requisite for man-less production and automation of the processes used for parts production.
Particularly, for processes like turning or drilling chip breakage is an important task, in turning especially when shaping tools are used and the chip formation cannot be influenced by chip breaking elements in the tool. In drilling, deep holes frequently make high demands on chip breakage of the material. Considering the remaining two main evaluation criteria for machinability, it can be stated that both process forces and surface quality are important for machining of free cutting steel. Normally the process forces are comparably low but when special tools and machines with low rigidity are used, even low forces can deteriorate the stability of the cutting process, and hence the quality of the product.
Besides the dimensional quality, the surface quality is also important since the parts are normally finished by machining with geometric defined cutting edge and do not need finishing operations like grinding or polishing. In addition to the main machinability evaluation criteria such as tool life, chip form, cutting forces, and surface quality, some phenomena influencing these evaluation criteria have to be considered exceptionally. Since machining of free cutting steels is frequently performed on multi-spindle machines and auto lathes, the used rotational speeds are low.
In combination with small work piece diameters, this results in low cutting speeds. Subsequently, the formation of BUEs can have strong impact on tool life and surface quality. Further, the cutting temperatures have to be considered since chip formation, tool life, cutting force, and dimensional quality have dependence on the cutting temperatures. Temperatures on the rake of the tool can also give information about the tribological conditions in the contact zones of tool and work-piece.
Another important phenomenon is the layer formation. Build up layers from different chemical compositions of edge and rake faces of cutting tools can result in a reduction of cutting forces and temperatures and hence reduce tool wear. Fig 1 summarizes the different criteria for evaluating the machinability of low carbon free cutting steels.
Fig 1 Evaluation criteria for machinability
Concept of free cutting steel – The mass production of the products makes large demands on the automation of the production processes in order to decrease cycle times and hence the production costs. As a result, reliable and efficient cutting processes are necessary. The machinability of the steel materials plays an important role since normally 42 % to 67 % of the total production costs originate from cutting processes. Hence, those work-piece materials are necessary, which allow for the exploitation of the technological limits given by the machine tools, production processes, as well as, the cutting tools and materials.
The arising requirements of the steel materials can be summarized as (i) tool wear as low as possible at cutting times as long as possible, (ii) short, lightly breaking chips for ensuring undisturbed chip removal (iii) sufficient surface quality, and (iv) low cutting forces and temperatures for allowing the application of sensitive cutting tools and machines.
The machinability of free cutting steels was a big issue of the 20th century. Hence, it was shown that the most important elements with respect to machinability improvement were sulphur (S) mainly in combination with manganese (Mn) and lead (Pb). Hence, the study focus was on investigations to describe the influencing factors on the cutting performance of resulphurized low carbon free cutting steels. In 1897 it was discovered that S is able to improve machinability and hence since 1903 low carbon (C) free cutting steel with S additions has been produced. Certainly, S is only slightly soluble in steel and hence the addition of only small quantities results in the formation of iron sulphide (FeS), which has a melting point of only 988 deg C.
When cooling down melted steel, FeS settles at the solidified crystals. When heating steel, the grain boundaries weaken in a temperature range of 800 deg C to 1,000 deg C and this leads to red brittleness, which implicates possibly failure in warm forming processes. At the temperatures above 1,200 deg C, FeS liquefies and causes hot breakage. In order to prevent this damaging effect, Mn is also added in the steelmaking process. It has a stronger affinity to S and hence forms manganese sulphide (MnS), which is characterized by a high melting temperature of 1,520 deg C. The sulphides solidify prior to the remaining melt as round inclusions, which act as seed crystals.
The Mn content is to be 4 times to 5 times that of the S content (Mn = 2.5 x S + 0.15) to avoid the formation of FeS. Normal S contents lie between 0.1 % and 0.4 % and is not to be higher since MnS deteriorates warm rollability. The influence on tensile strength and micro-structure is irrelevant, but the marked anisotropy impairs toughness in transverse direction.
Since the properties of the material of the work-piece are mainly determined by the purpose of use, the possibilities regarding modification of grain boundaries, slag content, cold working, or chemical composition are limited. The best approach to meet the mentioned requirements is represented by the big group of free cutting steels. The desired material properties with respect to machining processes are mainly achieved by alloying with S and Pb and further by adding other elements like phosphorus (P), tellurium (Te), selenium (Se), calcium (Ca), antimony (Sb), bismuth (Bi), or tin (Sn).
Free-cutting steels are used in a broad variety of applications, mainly in the automobile industry. These steels are used in the production of axles, shafts, connection rods, fitting turn-offs, high-pressure fuel injector parts, screws, bolts, and fittings. This group of steels results from the requirement to automated machining, i.e., they are easier to machine than classical steels resulting in higher cutting speed, lower tool wear, better surface finish, better chip breaking, and lower energy consumption of the machines.
Since machinability improvement is not only important for mild steels, free cutting steels can be classified respecting their heat treatment. As per this classification, free cutting steels are (i) low carbon free cutting steels (mild steels), which are not subject to heat treatment after machining (e.g., 11SMn30, 11SMnPb30, and 11SMnPbTe30 grades), (ii) free cutting case carburizing steels with a low carbon and partially low S content (as the name implies, these steels are used for case hardening after machining, examples are 10S20, C15Pb, 16MnCrPb5, and 16MnCrS5Pb grades), (iii) free cutting carbon (quench and tempering) steels characterized by carbon contents exceeding 0.3 % with majority of steels of this group showing low S contents in order to avoid substantial impact on material strength and toughness (e.g., 35S20, C35Pb, and C45Pb grades).
The EN 10087 standard describes free-cutting steels as those steel grades which normally have a minimum S content at least equal to 0.1 %. Depending on the material requirements, free-cutting steels are classified into (i) soft free-cutting steels, (ii) free-cutting hardening steels, and (iii) free-cutting heat treatable steels. Tab 1 shows the chemical composition of these steels as given in the European standard EN 100087-1998.
|Tab 1 Steel grades and chemical compositions of free cutting steels|
|Steel designation||Chemical composition in percent|
|Soft free-cutting steel|
|Free cutting hardening steels|
|Free cutting heat treatable steels|
|The elements not mentioned in this table should not intentionally be added without the customer’s agreement, except of those that are destined to the steel elaboration. Nevertheless, it is allowed for the producer to add elements such as: Te, Bi, etc. in order to improve machinability, providing that it has been agreed upon during tendering and ordering.|
|*: If the metallurgical techniques used could guarantee specific oxides formation, it is possible to accept a silicon content between o.1 % and 0.4 %.|
|Note: 1. Max – maximum, 2. Reference EN 10087-1998 standard|
It is well known that S reacts with Mn to form MnS inclusions. These inclusions soften during the cutting process at medium cutting speed (Vc) having a value of 150 metres per minute (m/min) to 300 m/min, where the temperature is locally higher than 700 deg C. At these temperatures, MnS inclusions are more malleable than the steel matrix, and help the chip breaking because of a shear localization around these malleable sulphides. It allows a better surface finish and avoid chip evacuation difficulties. Also, a thin MnS layer forms at the tool-chip interface acts as a lubricant and decreases the friction coefficient. It allows to increase the cutting speed for a given tool life or to increase the tool life for a given cutting speed. Hence, the productivity is increased.
Pb acts with equivalent mechanisms than those of MnS inclusions. But the small nodules of lead (5 micrometers), dispersed in the matrix, or at the tips of MnS inclusions as shown in Fig 3a, present a very low melting point (327 deg C). Hence, their beneficial effect on machinability acts even at low cutting speed (Vc less than 100 m/min). The cumulative effects of these two routes have been the heart of the low carbon free cutting steels development.
The shape of sulphide inclusions is basically defined by the oxygen (O2) content dissolved in the steel during solidification. Three different types of inclusions as described below are there.
Type 1 inclusions form as liquid phase in iron rich steel melts containing oxygen contents higher than 0.2 % as per the system Fe-MnO-MnS (iron-manganese oxide-manganese sulphide). Typical examples are rimmed or semi-killed steels with very low content of aluminum (Al) and silicon (Si). After solidification, these sulphides are randomly dispersed and show a globular or irregularly rounded shape. The inclusions are brittle and exist individually in closed segments. As a result of the high O2 content, these sulphides show low deformability, which makes decisive impact on machinability.
Type 2 sulphides can be found in steel melts containing O2 contents lower than 0.1 %. This type is normally present in killed steel. It segregates in a eutectic like MnS-phase. Type 2 inclusions are not to be found in closed segments but rather as fine sulphides in a fan or chain like pattern, actually precipitated as inter-connected branched rods.
Type 3 originates at very low O2 contents in iron melts with low melting point. These sulphides only occur when excessive quantities of deoxidizers are present. They are randomly dispersed and show angular or faceted shapes. The size and spacing of each of these morphological types can vary, depending on the solidification rate.
When the cast steel is subsequently hot worked, Type 1 sulphides tend to deform less than the matrix does and hence assume a somewhat elliptical shape. Type 2 sulphides, already fine and with elongated morphology, deform because of hot working and form clusters of long, thin particles along the principal deformation direction.
Machinability improvement – Sulphide, oxide, and Pb are typical improvers of machinability in the free cutting steels. These are described below.
Manganese sulphide (MnS) – In free-cutting steels, S is added up to 0.3 % to 0.35 %. During the cutting process, MnS inclusions are normally more plastic than the steel matrix, deforming preferentially and hence reducing the total stress involved in the chip formation. This results in reduced chip thicknesses and cutting temperatures. The presence of this easily sheared phase also promotes short chipping behaviour. The interaction of the free cutting additive with the tool material is also an important factor.
It is claimed that with the carbide tools, a layer of MnS forms on the rake face. It acts as a lubricant, and as a diffusion barrier. This reduces the tool-chip contact area, and causes a reduction in chip thickness, tool forces, and tool temperature. An increase of the chip flow rate for a given tool life is observed. It is well established that MnS improves machinability even at low cutting speed (Vc less than 100 m/min). But the optimal improvement is reached for medium cutting speed (Vc between 150 m/min and 200 m/min).
As the interfacial temperature approaches 700 deg C, ferrite tends to become less plastic than MnS and this can be emphasized, if the MnS inclusion has a high aspect ratio. With an austenitic stainless steel, if the cutting speed is increased still further, and inter-facial temperatures begin to exceed 850 deg C, austenite tends to become more plastic than MnS, the advantage is lost. Under these circumstances MnS becomes ineffective and can even increase the flow strength.
At higher cutting speeds (Vc higher than 200 m/min) while using cemented carbide tools, the seizing phenomenon sets in at the tool-chip inter-face, resulting in dissolution crater wear by diffusion mechanism. Sulphide inclusions are not effective in suppressing the dissolution wear by diffusion mechanism. MnS have also marked adverse effect on other properties of steels, especially the transverse impact toughness and fracture toughness. The MnS precipitation can leads to steel decohesion during cold forging. S also decreases the hot ductility, the forgeability, and the hot rolling behaviour of steels. The plasticity of MnS allows them to deform along the rolling direction. Hence, the mechanical behaviour in a perpendicular direction, is altered. Weldability and corrosion resistance are also decreased.
Main effects of MnS in cutting result in a reduction of process forces, cutting temperature, and chip thickness. The presence of MnS causes a movement of the cutting speed range where BUEs occur towards higher cutting speeds. However, regarding the function of MnS during cutting, partly oppositional explanations exist as given below.
There is reduction of the friction coefficient in the contact zones because of MnS layer formation. In contrast, a study reported as a result of friction tests that MnS shows poor lubrication properties.
There is reduction of friction because of the crystalline structure of the MnS metalloid. In the lattice the S atoms are located in a hexagonal plane, which are likely to slide over each other.
In the shear zone MnS operates as stress raiser. The sulphides can be regarded as voids, which lead to embrittlement of the material and support the initiation of micro-cracks within the primary deformation zone (PDZ). As a result, flow stress is reduced.
There is reduction of tool wear since MnS performs as a diffusion barrier.
There is decrease of inner friction and reduction of flow stress of the steel material of the work-piece. Correspondingly, there is reduction of friction in the contact zones because of the lower normal forces. The friction reducing effects of MnS are of vital importance and can be specified as described below.
Under the majority of the machining conditions, the contact zone between chip and tool rake can be distinguished into two regions. The region close to the tool tip is characterized by seizure and the rear part of the contact zone is dominated by sliding. Fig 2 shows these areas and further shows the distribution of normal compressive stress (S) and tangential shear stress (t) acting on the rake of the tool. The normal stress decreases from its maximum Vmax to zero from the tool tip to that point where the chip leaves the rake (A – C). A lot of studies have been done to investigate the exact distribution of normal and tangential forces on the cutting tool, but a very simple distribution as proposed in Fig 2 is sufficient for the explanation of the effects because of MnS inclusions.
Fig 2 Distribution of normal and tangential stresses
The shear stress ‘t’ can be calculated in the sliding region (B – C) by means of the normal stress and the friction coefficient ‘M’ given by equation 1 which is ‘t = M x S’ (sliding). and in the seized area (A – B) by the equation 2 which is ‘t = ts’ (seizure), where ‘ts’ is the flow stress of the chip material, which is reached in the secondary deformation zone (SDZ) at the bottom of the chip. Hence, ‘t’ becomes independent of normal stress, depth of cut, and rake angle.
Hence, for the seized region, a maximum friction coefficient ‘Mmax’ can be calculated by means of the shear flow stress according to von Mises as given in equation 3 which is ‘t = S/root 3’ and equation 4 which is ‘Mmax = ts/S = 1/root 3 = 0.577’. The total tangential force parallel to the rake is proportional to the area under the ‘t-x’ curve (Fig 2). Hence, the cutting process can be improved if ‘t’ or respectively the seized region is being reduced.
On the one hand this is realized by MnS inclusion effects (reduction of flow stress) in the primary deformation zone, which results in lower normal stresses and because of the mentioned correlation with the friction coefficient in a reduction of ‘t’. On the other hand, the tangential force can also be affected directly through the actions of MnS inclusions near the tool-chip interface. The reduction of the friction coefficient reduces ‘t’ in the sliding region and it causes also a decrease of the seized area, which finally results in a decrease of the tangential force.
Further, MnS inclusions can become highly elongated parallel to the rake in the secondary deformation zone and can form surfaces of weakness. Hence, they can reduce flow stress in the SDZ, which again contributes to a reduction of the tangential force. For achieving the mentioned effects of MnS inclusions during machining, different studies have placed emphasis on the influence of shape, distribution, dimension, and deformability of non-metallic inclusions on the machining performance.
It is pointed out, that globular sulphide inclusions of type 1 showing low deformation ability have a positive impact on the cutting process. If a sulphide inclusion is deformed as much as the steel matrix containing it, plastic flow in the material is homogenous and undisturbed. The role of MnS inclusions as a stress concentration inducer is then be minimal. Further, possible shear concentration effects at the interface between non-deforming inclusions and a ductile matrix is absent. Recent studies show that the decisive influencing factor regarding machinability improvement is the quantity of sulphide inclusions per volume rather than the size of the inclusions. Further, the study has exposed that machinability improvement because of long stretched sulphide inclusions of type 2 is also possible.
One of the studies has investigated in detail the influence of the steel production process, S and Pb additions, as well as cutting fluid on different work-piece areas. Further this work contained the evaluation of different machinability testing methods in order to show which short-term tests are profitable for machinability testing. However, it has been shown that the most important measure for improving machinability is the addition of Pb. Main benefits from adding lead can be summarized as (i) reduction of tool wear as a result of decreasing friction, lower cutting temperatures, and a decrease of cutting forces, (ii) movement of undesirable BUE formation towards higher cutting speeds and simultaneously decrease to and stabilization of BUEs resulting in improved machining performance at low speeds, (iii) generation of strongly curved, and short breaking chips, and (iv) improved surface quality particularly at low cutting speeds.
Oxides – Normally, free cutting steel grades process present a de-oxidation by Al in order to meet the requirement of the customers in terms of grain size and fatigue resistance. This process leads to the formation of hard alumina oxide inclusions detrimental to machinability, especially regarding the tool life of tungsten carbide (WC) at high cutting speed. One way to improve machinability is to decrease the abrasive nature of oxides by optimizing the steel refining process to get oxides with low melting points. For this purpose, a Ca treatment is normally carried out in the ladle and CaO-Al2O3-SiO2 (calcium oxide-alumina-silica) inclusions are designed. As an example, the glassy anorthite phase on the CaO-Al2O3-SiO2 phase stability diagram is deformable because of its low viscosity, which is lower than that of steel.
Glassy inclusions are designed to soften at the tool-chip interface temperature and form a viscous layer, so that the shear is accommodated within the viscous layer. Hence, the viscous layer of glass lubricates the tool-chip interface, thereby preventing the occurrence of tribological phenomenon of seizing. It also acts as a diffusion barrier. Moreover, the soft inclusions deform preferentially in the shear band, hence reducing the total strain involved in chip formation. The distance between shear bands is more regular. The presence of this easily sheared phase also promotes short chipping behaviour.
Deformable glassy oxide inclusions such as CaO-Al2O3-SiO2 engineered in the work-piece, are found to be an effective means of suppressing dissolution crater wear of carbide tools at high cutting speed (Vc = 200 m/min). The wear rates of coated carbide tools are diminished to 20 % to 30 % with engineered inclusions compared to standard steels. The cutting speed can be increased in the same rate without increasing tool wear. The mechanism of machinability of the oxides is similar to that of sulphides. But these inclusions present a higher softening temperature, and are really plastic in the temperature range of 800 deg C to 1,200 deg C, or at high strain rate. This temperature can be reached only at high cutting speed (Vc higher than 200 m/min). Hence, machining with carbide tools is more likely to get the improvement by engineered oxides.
If high speed steel tool is used, the oxides improvement effects are visible only in particularly hard machining conditions, i.e., drilling with high feed rate as an example. This explains the major draw-back of engineered oxides. This type of inclusion does not improve machinability at low cutting speed (Vc less than 100 m/min) since the temperature reached at the tool-chip interface is not high enough to soften the inclusions and they are not malleable enough.
Oxide-sulphide synergy – After a Ca treatment on high S content steels, an interaction involving (Ca,Mn)S inclusions and CaO-Al2O3-SiO2 inclusions is observed. Oxide inclusions are enveloped by a sulphide shell. The abrasive behaviour decreases considerably. The synergy of the sulphides and malleable oxides effects leads to 45 % increase of the productivity.
Lead (Pb) – In low carbon free-cutting steels, Pb additions, around 0.3 %, improve machinability because of (i) better chip breaking, (ii) longer tool life, and (iii) better surface finish and dimensional tolerances.
Normally in steels, Pb contents up to 0.35 % are believed to improve machinability, whereas the dependency of the wear behaviour of leaded steel is to be analyzed in more detailed. Pb and S content are decisive as the degree of improvement because of Pb diminishes with increasing S content. Increasing Pb content results in a strong improvement at low cutting speeds, whereas at higher speeds a deterioration of the cutting performance is likely. However, the influence of Pb additions on tool wear has to be distinguished regarding the wear form.
One of the studies has shown that because of alloying with Pb (9SMnPb23 grade), edge wear can be reduced by around 40 %, whereas crater wear increases by around 33 %. These higher values for crater wear can be justified with rising surface pressure. Since the cutting force decreases not as much as the length of the contact zone, the surface pressure is presumed to rise, which results in stronger crater wear. The increased wear attack on the rake because of alloying with Pb is confirmed in extensive machinability tests in a study. In turning, at the whole cutting speed range of Vc = 80 m/min to 140 m/min, 9SMnPb28 grade steel causes stronger crater wear compared to Pb free 9SMn28 grade steel and a considerable influence of cutting speed is not visible.
On the contrary, when analyzing edge wear, it can be realized that the major influencing factor on the wear behaviour of leaded steel is the cutting speed. The drop of tool wear with increasing cutting speed is larger for leaded steel and correspondingly the Taylor straight lines of 9SMnPb28 and 9SMn28 grades intersects at a cutting speed of around 100 m/min. This indicates, that tool life in cutting leaded 9SMnPb28 grade steel is higher at low cutting speeds, whereas at increased cutting speeds more tool wear occurs compared to 9SMn28 grade steel. This correlation is not depending on the application of cutting fluid.
The main reason for the reduction of edge wear at cutting speeds below 100 m/min can be seen in the formation of BUEs. Leaded steels show considerably smaller dimensions of BUEs and the frequencies characterizing the release of BUEs from the tool are lower. In contrast, the temperature reducing effect of Pb results in BUE formation at cutting speeds, where in machining non-leaded steels BUE no longer occur. More recent studies on free cutting case carburizing steels (16MnCr5Pb and 16MnCr5S steel grades) confirmed the results. Using uncoated carbide inserts in a dry cut, an increase in machined material volume can be achieved for the leaded steel at Vc = 200 m/min, whereas at Vc = 340 m/min a reduction by 12 % is ascertained. Similar results can be achieved in turning with coated carbide tools.
Majority of the studies regarding the performance of Pb are focused on turning experiments but it is also reported on improved machining performance in drilling processes. These tests confirmed the machinability improvement behaviour of Pb as the torque is reduced and the maximum number of bore holes until tool failure can be increased. The understanding that alloying with Pb is the best way to improve machinability has been realized a long time ago, whereas newer studies have dealt with possible alternatives to Pb as machinability improver.
Pb is not soluble in solid steel, where it is found in the form of small spherical particles. Good leaded steel shows an ‘emulsion’ of small Pb particles of an average size of 6 micrometers to 7 micrometers either free or attached to MnS inclusions (at the tip of sulphides) as shown in Fig 3a.
Fig 3 Observations of leaded steel and hot ductility of free cutting steels
The mechanisms which have been reported to explain how lead improves machinability are described below.
Lubrication is the most widely accepted effect of lead on machinability, since lead reduces the friction at the tool / chip interface. Here, temperatures are above the melting point of Pb and Pb films are formed on the underside of the chip and on the tool surface, being an effective lubricant.
During the cutting process, lead inclusions (similarly to MnS) are normally more malleable than the steel matrix, deforming preferentially and hence reducing the total stress involved in chip formation. This is reflected in the increase of the shear angle, in reduced chip thicknesses, and cutting temperatures.
It is suggested that different mechanical properties between the soft additive Pb particles and the matrix improve stress concentration by gap formation and micro-cracking at the interface. The improved machinability is suggested to occur by the reduction in the effective area available to resist to shear stresses in the deformation zone, hence lowering the cutting forces and power consumption.
During hot-ductility tests, some studies discovered that an embrittlement trough in the temperature range from around 200 deg C to around 600 deg C occurs, in which the fracture mode changes from a relatively ductile mode, to a brittle intergranular mode (Fig 3b). The fracture analysis of unleaded 11SMn30 grade steel indicates that this steel only shows ductile fracture over the entire range of test temperatures. Pb causes this embrittlement by being present at, and weakening, the ferrite grain boundaries. More precisely, Pb lowers the grain boundary cohesive strength. The energy necessary during cutting decreases, and the machinability is improved. The presence of an easily sheared phase also promotes short chipping behaviour, which is probably the major advantage as far the machinist is concerned. In the shear band, MnS and MnS-Pb inclusions behave differently (Fig 4). In the case of Pb free MnS inclusions, the gaps reweld because of the high compressive stress and high temperature. Hence, the gaps do not remain in the chip. In the case of MnS-Pb inclusions, the Pb prevents the voids from rewelding, hence forming the desired small chips. Hence, Pb improves the effectiveness of MnS inclusions by either restricting deformation of MnS or weakening the inclusion-matrix interface, and stabilizes the chip fracture process.
Fig 4 Behaviour of different types of inclusions during machining
Liquid metal embrittlement (LME) is the loss of because of the presence of low melting point phases. If a crack is induced in the primary shear zone, the liquid lead flows to the crack tip reducing the binding energy of the atomic bond. This results in nucleation and propagation of the crack at a lower stress level. Since for normal cutting speed, the temperature in the secondary shear zone exceeds the melting point of Pb, it is assumed that LME plays an important role in reducing the force in the secondary shear zone, hence improving machinability and chip fracturing in particular.
n unstable BUE can form on the rake face near the tool tip, when machining under unfavourable cutting conditions (very low cutting speed, less than 100 m/min). Some studies have shown that tool wear is even higher with leaded steel at high cutting speed (Vc higher than 200 m/min). Also, Pb is toxic and conducts to recycling and environmental issues, and hence special fume cleaning installations are to be used in the steel plants together with a restriction in the man-power exposure to Pb fumes.
The regulations in some countries on end-of life vehicles prohibits the recycling of vehicle components containing heavy metals including Pb. For the time being, free-cutting steels with up to 0.35 % of Pb are exempted from this ban but the use of Pb is being periodically reviewed taking account of the scientific and technological progress.
Pb suppression and improvement of sulphides – MnS inclusions improve machinability, this aspect has been developed previously. In order to suppress Pb in free-cutting steels, improvement of sulphides existing in the steel can be an interesting concept.
Increase of sulphur content – Free cutting steels normally contain sulphides of type 1 and type 2 (Fig 5a). For optimizing the machinability of free cutting steel, coarse, slightly deformed inclusions of the first type are to be the predominant type of inclusion in the steel. An increase in the content of such inclusions is accompanied by a decrease in the degree of the adhesive interaction between the material of the work-piece and the material of the tool (Fig 5b), and, hence, a decrease of the tool wear rate. Hence, it is considered to increase the quantity of S from 0.3 to 0.37 % (11SMn37 grade) in an unleaded steel.
Fig 5 Manganese sulphide inclusions and effect of weakly deformer manganese sulphide
The higher number of sulphide inclusions improve machinability more than in conventional 11SMn30 grade steel. This improvement is because of a reduction of the tool wear rate, assuming that the cutting speed is quite high to cause a tool temperature at the tool-chip interface which provides malleable MnS. But this improvement is small at low cutting speed (Vc less than 100 m/min) where Pb plays its best role (because of a low melting temperature). The steel shows slightly improved performance compared with the 11SMn30 grade steel in component production tests using high speed steel tools, and lubricant. It also shows good chip formation, but it does not approach the leaded steel (11SMnPb30 grade) in surface finish.
Morphology of sulphides – In order to lower the plasticity of the sulphides, and to keep them globular during hot rolling and avoiding perpendicular defects, tellurium (Te), selenium (Se), and calcium (Ca) can be added. The combination of increased S with Te in free-cutting steels gives an improvement of machinability in surface finish. Although this steel does not approach the performance of the leaded steel in terms of production rate, it shows a better machinability, than conventional unleaded steel. Te forms low melting phases like MnTe and FeTe or PbTe in leaded steels. These phases are found mainly around the MnS and cause the sulphides to deform less during hot rolling. Hot workability is severely affected. For this reason, the addition of Te is normally restricted to below 0.01 %.
The influence of Se is similar to that of Te, its price being higher. It forms with Mn, MnSe compound which is, in solid state, completely soluble with MnS. MnSe helps MnS to stay globular. For getting the same effect as with Te, Se is to be added in double quantity. It does not deteriorate the hot-rolling behaviour as much as Te, but its toxicity prevents a wide spread use in steels. Te and Se are normally added to leaded free-cutting steels but are not considered as substitutes. Indeed, no effects on machinability are expected at low cutting speed.
Calcium and rare earth (RE) metals have been added in resulphurized free cutting steel to study their influence on MnS and machinability. Tests in turning have been performed with high-speed steel (HSS) tools at low cutting speed, between 5 m/min and 25 m/min. The behaviour of the sulphide inclusions during machining depends mainly on their deformability, which in turn, depends on their composition. The higher is the deformability, the lower is the cutting forces. The deformability of sulphide inclusions can be presented in this decreasing order, MnS, (Mn,Ca)S, MnS-RE2S3 and (Mn,Ca)S-RE2S3. Hence, the cutting forces are reduced in the same order, more reduced by MnS inclusions than by (Mn,Ca)S-RE2S3 inclusions.
Non-deformable inclusions promote unclosed micro-voids in the steel matrix which are helpful for the reduction of the edge wear of the cutting tool. The tool wear is reduced in the increasing order, MnS, (Mn,Ca)S, MnS-RE2S3 and (MnS,Ca)S-RE2S3. But the correlation between unclosed micro-voids and tool wear is not available. A Japanese patent suggests to add simultaneously Ca and magnesium (Mg) to S to form sulphides and soft oxides in order to improve the machinability. It also suggests to decrease the O2 content to 20 ppm (parts per million) to avoid the formation of abrasive hard oxides. The edge wear of carbide tools in turning at Vc = 150 m/min is equivalent to leaded steel. But the behaviour at low cutting speed is not presented in the patent.
The shape factor (l/t, l is length and t is thickness) of sulphide has an interesting effect on machinability. It has been seen that in Ca treated sulphide, a higher shape factor (non-treated deformable sulphide) is more effective in reducing the cutting force, but less effective in reducing the edge wear of the tool in turning, compared to more globular sulphide (Ca-treated sulphide). There are two opposite factors regarding the effect of shape factor of sulphide inclusions on surface finish. On the one hand, a higher shape factor of the sulphide inclusions can lead more easily to the formation of a BUE, impairing surface finish. On the other hand, however, the micro-voids initiated by sulphide inclusions with higher shape factors can be more easily closed up, improving the surface finish.
The size of the sulphides plays also a role in machinability. It is known that machinability is improved when the size of sulphide inclusion becomes larger. A Japanese team has studied the formation of MnS with high S content and Cr (chromium). The study suggests to substitute Pb for the reasons namely (i) since Cr substitutes Mn in sulphide, an increase in the size of sulphide can be expected (it is since Mn and Cr atoms show nearly the same size, so the effect of Cr on sulphide size is to be equivalent to that of Mn, and not superior, (ii) (Cr,Mn)S also improves machinability, and (iii) Cr is not harmful to the environment. In the studied process, the size of sulphide is increased compared to conventional 11SMnPB30 grade (Pb – 0.2 % to 0.3 %) leaded steel.
The increase is more because of the increase of S content (0.385 % compared to 0.34 % for 11SMnPB30 grade than to the addition of Cr. Also, the quantity of sulphide is larger than in 11SMnPB30 graded leaded steel. It contributes to the refinement of micro-structure by suppressing austenite grain growth during reheating and by functioning as nuclei for the ferrite transformation. This refinement improves the machinability in the same manner as increasing the size of sulphide inclusions. The machinability, equal or superior to that of 11SMnPB30 grade, has been confirmed with carbide tools at high cutting speed in turning, and with HSS tools at low cutting speed in drilling. Similarly, the machined surface roughness of the developed steel is equal, or smaller, than that of 11SMnPB30 grade at all cutting speed. But the turning at low cutting speed (Vc less than 100 m/min) has not been studied, and it is not possible to conclude on the effectiveness of the Pb substitution in this speed range
Suppression of Pb, and enhancement of sulphides, is claimed to be effective in several cases. Mainly, the objectives are to keep sulphides globular and to increase their size to improve the machinability. The performance in machining, compared to leaded steel at low cutting speed (Vc higher than 100m/min) is rarely explored. When it is explored, it is normally not better than that of leaded steel, except for the last example quoted with (Cr,Mn)S inclusions during a drilling operation.
One of the studies concentrated on tool wear and showed a high potential of HfN coated cemented carbide tools with respect to tool life improvement in machining of Pb free steels. Further, in this study, it is pointed out that the shape of non-metallic inclusions is a very important factor in order to ensure good machinability of non-leaded steel.
In another study, different approach has been chosen to provide lead free steel with improved machinability properties as they replaced Pb by Sn (tin). It has been claimed that the decisive material behaviour of Pb, namely the tendency to embrittlement in certain temperature ranges, can also be observed for Sn. Production trials focusing on wear behaviour of leaded and tin added steel showed a considerable influence of tool material. Using high speed steel tools, leaded steel is superior, whereas Sn added steel shows better performance when cemented carbide tools are used. All machining tests have been conducted under oil lubrication. Further, milling tests with single point high speed steel tools has indicated better machining performance of Sn and Pb added free machining steels compared to Pb free steel. A steel with a lower content of Sn (0.04 %) has shown worse machinability compared to leaded material, whereas a higher Sn content of 0.08 % led to an improvement in machinability compared to leaded standard steel. Finally, the results indicated that the Sn alloyed steel can be machined at higher cutting speed.