Effect of Inclusions on the Properties of Steel
Effect of Inclusions on the Properties of Steel
Steel is a versatile material which is having very wide applications. It is of interest for several applications because of its several advantages such as high strength to weight ratio, durability, versatility, recyclability, and most importantly its economic viability in comparison to other engineering materials. Besides its common uses, it is also the material of choice for many industrial components used in critical applications. These critical applications demand very stringent requirements in terms of the steel properties. Such requirements vary in terms of their specific needs ranging from light weight, high strength, high toughness, ability to withstand high pressures, ability to withstand sub-zero temperatures, excellent weldability, good corrosion resistance, and more frequently than not a combination of such properties is needed.
The versatility of steel allows the engineer to tailor the properties by modifying the chemistry and / or the microstructure. Despite the fact that several developments have taken place with regards to these two variables, another crucial aspect which determines the performance in service of steel is how free of impurities it is (sometimes called cleanliness). Steel cleanliness is determined by the non-metallic Inclusions (or simply inclusions) which are embedded in it. In order to improve the performance of steels, inclusions are to be controlled since the inclusions are a critical problem of steels for structural applications and depending on their size, shape, and distribution, they can be very detrimental to the mechanical properties.
Inclusions are chemical compounds consisting of a combination of a metallic element (iron, manganese, silicon, aluminum, and calcium etc.) and a non-metallic element (oxygen, sulphur, nitrogen, and carbon etc.). The most common inclusions include oxides, sulphides, oxy-sulphides, phosphates, nitrides, carbides and carbo-nitrides. Depending on their nature and cooling conditions during the solidification stage they can present a crystalline or a glassy state. Inclusion form phases different to the steel although some represent a higher mismatch than others depending on their crystalline structure and atomic sizes. Inclusions containing more than one compound are called complex inclusions (spinels, oxy-sulphides, and carbo-nitrides etc.).
In terms of size, inclusions can be either micro inclusions or macro inclusions. The threshold value which has been used to distinguish between micro inclusions and macro inclusions is normally assumed to be 100 micrometers. However, more recently with the advancement of steelmaking practices to control the sizes of inclusions, another way to refer to micro inclusions has been proposed, which is namely the diameter sizes below their floatability limit and has a value which is in the dozens of micrometers for modern steel processes. Micro inclusions are the most abundant due to their small size and tend to be more uniformly distributed in the liquid steel, and hence are seen to be less harmful. Macro inclusions due to their larger size are responsible for the failure initiation in final products or defects on semi-finished products.
Inclusions influence several properties of steels relevant to their performance in mechanical and structural applications. Some of the harmful effects that presented by the inclusions in the cast steel can be reduced with hot working as this process can induce orientation changes and a break up of inclusions. Hence, the exploration of the different factors which affect the steel quality in terms of its fabrication and further processing together helps to better understand their relationship for ensuring consistent quality to comply with the evermore stringent mechanical property requirements of steel components for demanding applications. The understanding of how this happens has evolved in the recent past. Considerable progress in the quantification of this understanding has been made.
While a large emphasis is given to the importance of steel microstructure, the influence of inclusions on the steel properties is comparatively neglected. However, in industry the attention to the importance of inclusions on steel performance is there and the focus is for constant studies and improvements. This has become especially important as steel has been challenged by different alternative materials and by more demanding applications. The improvement of several properties only became possible with the understanding of their relationship to the type, size, and distribution of the inclusions present in the matrix. In the recent times, the steel industry has developed considerable process improvements which have led to much better control of inclusion volume fraction, size, and composition.
Tailoring inclusions to improve properties and performance is an important feature of steelmaking and the term ‘inclusion engineering’, coined in the 1980s is widely used. Inclusion engineering starts with the definition of the desirable properties which the inclusions need to have. Then, through the definition of adequate processing conditions, a product is made where these desirable inclusions are predominantly formed. Controlling inclusion distribution in the final product, in special those inclusions formed after the beginning of solidification (secondary inclusions) remains a considerable challenge.
Inclusions are inevitable chemical compounds embedded in the steel matrix, consisting of at least one non-metallic component, such as oxygen, nitrogen or sulphur. These compounds can originate at various stages of the steelmaking process. The role of the steelmaking process in terms of inclusion control is very important since the inclusions originate and can be modified at various stages along the process route. Further, inclusions are detrimental in the way that they break the homogeneity of the structure when it has solidified. The stages of secondary steelmaking which play an important role for inclusion control include deoxidation, desulphurization, vacuum degassing, and argon stirring. During these operations alloying agents are added, dissolved gases in the steel are reduced, and inclusions are removed and / or altered chemically for ensuring high-quality of steel. Fig 1 shows critical metallurgical reactors (ladle, tundish and mould) for inclusion control in continuous casting of steel.
Fig 1 Critical metallurgical reactors for inclusion control
After performing the operations of secondary steelmaking, the refined steel is then transferred into the continuous casting machine in order to cast the liquid steel into cast product (slab, bloom, or billet etc.). During casting, several different interactions between steel and inclusions can occur. Reactions between the casting powder and the liquid steel can take place, and entrapment of casting powder can occur. Submerged entry nozzle (SEN) design and fluid flow, electromagnetic stirring, and the use of a vertical or curved mould are some of the main phenomena having an impact on the final inclusion content of the steel. Fig 2 shows inclusion phenomena occurring in continuous casting process because of different interactions between steel and inclusions.
Fig 2 Inclusion phenomena occurring in continuous casting process
The entire process of inclusion removal in the liquid state consists of a ‘nucleation-growth-removal’ cycle. The formation of inclusions can be divided in different stages depending on phenomena which occur at each one of them. Nucleation occurs as a result of super-saturation of the liquid steel with the solutes due to a change in temperature or chemical composition of the system. The growth of inclusions continues until there is no super-saturation or chemical equilibrium is achieved. The motion of liquid steel due to thermal convection or magnetic stirring forces causes the coalescence or agglomeration of (liquid or solid respectively) inclusions. Inclusions with higher surface energy tend to merge more easily than inclusions with lower surface energy. It is easier to float the larger inclusions to the slag where they are absorbed, but this removal process depends on the particle radius. In Fig 3, the processes, phenomena and evolution mechanisms of inclusions at different stages of the manufacturing of steel are described.
Fig 3 Inclusion growth and removal mechanism
The inclusions can be of globular shape, platelet shape, dendrite shape, and polyhedral shape. In terms of their shape the most desirable is the globular shape because of their isotropic nature with regard to their effect on the mechanical properties. Platelet shaped or thin films are located at grain boundaries due to the eutectic transformation during solidification. These are the most harmful to mechanical properties since they weaken the bonds at grain boundaries. Dendrite shaped, are caused by an excess amount of aluminum. These inclusions have high melting point and can cause clogging in liquid stage. In the solidified steel the sharp edges and corners of the dendrite can cause concentration of internal stresses and negatively impact the mechanical properties. The polyhedral inclusions have a lower effect on mechanical properties than dendrite or platelet shaped inclusions because of their more globular shape. The morphology of dendrite shaped inclusions can be modified to polyhedral shape by small addition of rare earth elements (cerium, Lanthanum) or alkaline earth elements (calcium, magnesium).
For better understanding the behaviour of inclusions, it is necessary to understand the transition from the liquid to solid state for both the steel matrix and the inclusions. The physical properties of the surrounding matrix and the inclusion at solidification temperature are of importance, since they can present different scenarios. If the inclusion is liquid (i.e. with a lower melting point) at steel solidification temperatures a compressive residual stress system develops ensuring coherency between the inclusion and the matrix. On the other hand, if the inclusion is solid when the steel is solidifying, the stress development depends on the different thermal expansion coefficients of both species. When an inclusion contracts to a lesser extent than the matrix a compressive residual stress develops within the inclusion and a resultant tensile stress develops in the matrix around the inclusion. On the other hand, if the inclusion contracts faster than the matrix then tensile residual stresses are generated in the inclusion and decohesion of it and the matrix occurs in the form of a void.
Inclusions constitute a very small part of the solidified steel and are normally finely dispersed. They are detrimental in the way that they break the homogeneity of the structure. The deformation behaviour of inclusions during the hot working of steel is of great importance for the properties of the final product. Both the steel matrix and the inclusions are normally multiphase structures, but for the sake of comparison, steel can be regarded as a homogenous phase since the structures of the inclusion are coarser when compared to steel microstructures.
Internal stresses can generate due to inclusion and matrix thermal expansion differences. The effect of the steel matrix flowing over and around the inclusions generates the deformation of the inclusions and the degree of deformation decreases with the elongation of inclusions as a result of the friction at the interface in the direction of rolling. If an inclusion has a strong interfacial bond, the inclusion gets lengthen and remains unbroken during hot working. On the other hand, if an inclusion has a weak interfacial bond it does not interact with the flow of steel and discontinuities can be produced. From this point of view, inclusions can be categorized as (i) inherently plastic inclusions (such as manganese sulphide), (ii) non-crystalline glassy inclusions which behave rigidly but become plastic at some characteristic temperature (such as some glassy silicates), and (iii) crystalline ionic solids (such as calcium aluminates, aluminate oxides and some crystalline silicates) which show no plasticity and behave in a brittle manner.
Several elements which are acceptable in steel composition have a high affinity for oxygen and hence can be used as deoxidizers, forming non-metallic deoxidation products when added to the liquid steel. Examples are silicon, manganese and aluminum. Deoxidation products can become important oxide inclusions. In the case of sulphur, on the other hand, only elements with low solubility in iron (such as calcium and manganese) or rare earth elements have sufficiently high affinity for sulphur to form non-metallic sulphides at the liquid steel temperatures. Hence, majority of the sulphur in steel is to be removed from solution by slag refining and the rest, by precipitation reactions occurring mostly during solidification. The most common sulphide precipitating during solidification is manganese sulphide.
Based on these observations two possible classifications for inclusions which emerge are (i) using their chemical composition (oxides, sulphides, etc.), or (ii) considering the moment they form with respect to the start of the solidification such as primary, before solidification starts, and secondary, after solid steel starts forming in the mould. Also, inclusions originating from the steelmaking process are classified as ‘endogenous’ and those appearing from ‘external’ sources (fragments of refractories, entrapped slag, etc.) are classified as ‘exogenous’. Rarely however, an ‘exogenous’ volume of material survive long enough in the steel without suffering extensive reaction with the liquid steel. These reactions produce changes in the inclusions. Hence, this classification can, sometimes, be confusing.
Finally, a common way of classification of inclusions is related to the inclusion size by which inclusions can be classified as macro inclusions and micro inclusions. A sensible cutoff between sizes is that an inclusion is a macro inclusion if it is large enough to cause immediate failure of the product either during processing or use. All other inclusions are classified as micro inclusions. Hence, while important, this is a difficult classification to apply.
Some properties of inclusions have a key importance on how they influence the behaviour of steels. These include plasticity or hardness as a function of temperature, coefficient of thermal expansion (CTE), crystallization behaviour (in the case of glassy inclusions), and to a lesser extent, solubility of metallic solutes. Inclusions have ionic, covalent, or mixed bonding character. As such, they are in general brittle at room temperature and have no strong bonding to the metallic matrix. As temperature increases, some inclusions become more plastic.
Several measurements of hardness and plasticity of inclusions indicate that the observed changes are too complex to describe in a simple way. One of the most widely used concepts to describe deformation behaviour continues to be inclusion ‘relative plasticity’. This concept is especially useful when there is a lack of accurate knowledge of the properties of the inclusions. The concept of relative plasticity which was introduced in the 1960s indicates the ratio of the true deformation of inclusions to the true deformation of the steel. Depending on temperature and inclusion composition, the relative plasticities of inclusions vary, and the inclusions can deform, crack, or have mixed behaviour. This is shown in Fig 4.
Fig 4 Behaviour of inclusions during deformation
As shown in Fig 4, the combination of low bonding strength to the matrix and matrix deformation leads to void creation and separation (or debonding). Also, hard inclusions typically break and redistribute in the steel under these conditions. This has been discussed taking the case of alumina. Understanding the breakage and redistribution of alumina is further complicated by the fact that alumina inclusions frequently cluster during the processing in the liquid state. Prediction of the behaviour of hard inclusions or inclusions which become less plastic at lower temperatures has been a challenge.
However, there are basic parameters needed to quantitatively describe the mechanical behaviour of inclusions and hence their effects on steel properties. Inclusions which are plastic at the working temperature deform when steel is worked. This results in the elongation of the inclusions along the major working directions. This introduces, in several cases, shape anisotropy in the inclusions. This results in anisotropy of the properties influenced by the inclusions. Recently the relationship between high temperature plasticity and melting point of oxide inclusions has been confirmed, by correlating the calculated liquidus temperature of the inclusions with their measured aspect ratio.
In some modelling work of inclusions deformation during hot working, the plastic deformation of the inclusions is associated with its viscous flow and reasonable prediction of anisotropy is achieved. In a quantitative study, it has been shown that the ratio of flow stress of the inclusion to that of the matrix defines the elongation of the inclusions. The behaviour of composite inclusions, having alumina surrounded by manganese sulphide is as shown in Fig 4(d). The results confirmed the experimental observations which show that as the sulphur content falls below 60 ppm (parts per million), the aspect ratio of the alumina / manganese sulphide inclusions decreases substantially. These results are of special importance for modern high purity, clean steels.
The anisotropy introduced by the inclusion shape change cannot be eliminated by further heat treatment. The deformation of inclusions and of segregates is normally responsible for the ‘fibre’ appearance observed during the macrographic examination of steels. An eventual crystallization of the inclusions during the steel processing can complicate the prediction of the extent of their deformation and the variation of plasticity with temperature. Inclusions which are initially ‘glassy’ or amorphous have been shown to crystallize when subjected to treatments at temperatures in the range of hot working temperatures of steels. The crystallized inclusions have different rheological behaviour (or relative plasticity) when compared to the ‘glassy’ Inclusions. This can be especially important when the plasticity of the inclusions is critical.
The coefficients of thermal expansion (CTE) of inclusions are different from that of steel. The differences in CTE can influence residual stresses around inclusions. Inclusions which can be more harmful by being surrounded by a tensile stress field associated with the tessellated (denoting or characterized by a pattern of repeated shapes, especially polygons, which fit together closely without gaps or overlaps) stresses. This can be especially important under fatigue conditions. These stresses are also considered relevant to machinability. It is believed that the formation of stress fields, cavities and pores in the steel matrix around the inclusions has a favourable effect on machinability. Inclusions having higher CTE than the steel separate from the matrix on cooling from steel processing temperatures. This can also cause problems in the metallographic sample preparation and difficulties with size determination.
When automatic methods are used, a gray-level threshold is set to differentiate between oxides and sulphides. Depending on the selected threshold, the dark region between matrix and inclusion can contribute differently to the measured inclusion size. Considerable differences exist in the use of the expression ‘inclusions’ in the discussion of fracture modelling. Some includes carbides, carbo-nitrides and other second-phase particles in the definition of ‘inclusions’. When considering the ductile fracture process, it seems important to take in account both type of particles. Special attention is required to be paid to the difference of the strength of the matrix-inclusion interface, and the size and distribution of the different types of particles.
The interface between the inclusions and steel has, in general, very low or no strength. On the other hand, the strength of the interface between carbides and steel, for example, has been estimated in the range of 1,200 MPa to 2,000 MPa. This difference has great importance on the effect of inclusions on the steel properties, particularly fracture. The distinction between the inclusions and second phase particles can become blurred. In some steels, titanium is used for the fixing of nitrogen, and titanium nitride inclusions can be formed in the liquid state. On the other hand, in electric steels manganese sulphide is formed as a fine precipitate to control grain boundary movement. In a study, using alumina particles in steel, it has been demonstrated the controlling role of particle size and volume fraction in affecting grain boundary movement.
Influence of inclusions on the properties of steel
The influence of inclusions on the properties of steel is being studied since a long been time. Inclusions can be tailored from the steelmaking process for as cast products to improve the steel properties. Also by knowing the required performance of wrought products, the inclusions of a certain grade of steel can be ‘engineered’. In order to properly address the improvement of the steel properties by means of inclusion engineering, it is important to have knowledge of phenomena and behaviour of inclusions along the entire processing route, from the liquid stage through to the post casting operations and their effects in wrought products.
In 2009, the European Commission published a research review comprising studies carried out at 4 major steel producers, the aim was the optimization and evaluation of different secondary metallurgy routes to achieve high-quality strip steel by controlling the inclusions, where for the production of bulk materials it is a matter of reproducibility, whereas for special steels is individually tailored. Hence it is important to understand the effects of the route on inclusions populations and the effect which inclusions have on the steel properties.
A recent study on the effect of inclusions on the steel properties has been made on the critical measurements in modern steelmaking to assess the influence of process conditions on product properties of carbon aluminum killed steels, medium carbon aluminum killed steels, advanced high strength steels, and free machining steels, all these taking into account the stringent requirements of steel properties for automotive applications, which include low inclusion content and calcium modification to ensure higher formability and improved mechanical performance of automotive parts.
The requirements for cleanliness with respect to the inclusions vary from product to product. There can be no universal definition of cleanness with respect to inclusions. The requirements is to be considered with respect to the demands of the specific application which the steel is going to be used for and also for many other aspects like their location, shape, and distribution in the steel component.
Effects on processing (hot and cold working and forming) – It is sometimes convenient to separate the influence of inclusions during processing from that during application, as the conditions in processing are normally not the ones envisaged for the steel application. This is true even considering that majority of the issues associated with the inclusions are related to their relative plasticity and their influence on steel ductile fracture. Inclusions which occupy a considerable portion of the material cross section during hot or cold work or which are in regions where processing deformation is high can cause fracture during processing. Hence, the control of the volume fraction, size and distribution of inclusions is important. Also, inclusion engineering is important for the fatigue properties and that the reasonable prediction of anisotropy is achieved.
Important areas which present challenges to further advances are (i) the proper characterization of the properties of inclusions and their interfaces with steel, (ii) an adequate metric for characterizing size, shape, and distribution of the large multi-particle population, and (iii) the difficulties associated with computational methods considering the multi-particle population.
Influence on the tensile strength – The tensile strength of steel can be affected by the final volume fraction of inclusions, and also the morphology and orientation of inclusions. The orientation of inclusions with respect to the direction of loading is of importance because of the fact that certain inclusions levels affect the ductility of the material. In the case of smaller cross-sections, the effect of inclusions is higher due to the role of inclusion sizes acting as nucleation sites of micro voids either by decohesion with the matrix or by fracture of the inclusion, which negatively affect the ductility of the steel. The inclusion volume fraction levels in the present steelmaking practices have been considerably reduced, to the point that their effect on the tensile strength is practically negligible in standard testing sizes. In a study which has investigated steels with various impurity levels to determine the tolerance levels to inclusions in ultrahigh strength steels, it has been found that while elongated manganese sulphide reduces ductility and bendability, however they does not have any notable effect on the strength.
Effect on toughness – Fracture toughness is the property of a material to resist the propagation of a crack, and is a crucial property used in the design of several engineering components. Majority of the inclusions are considered as stress raisers in the solidified structure and can cause failure by means of fracture. The distribution of void nucleating particles is considered as involving two size scales namely (i) larger inclusions which nucleate voids at relatively small strains, and (ii) smaller particles which nucleate voids at much larger strains. The nucleation of a small crack normally happens at larger sizes of inclusions and the propagation of the crack happens through linkage of micro voids created at smaller inclusions. The size of the void nucleating particles is typically between 0.1 micrometers to 100 micrometers, with volume fractions of no more than a few percent although this small percentage plays a major role in the crack growth resistance of structural alloys. The fracture modes in steels consist of three main different mechanisms as given below.
- Cleavage – It is a trans-granular fracture mode in which fracture propagates through crystallographic planes inside grains and the fracture surface appears as a series of flat planes. In this fracture mode, the main way to improve toughness is by controlling the microstructural unit which produces the propagation planes, which in ferritic steels it is the ferrite grain size and in pearlite and bainite it is the prior austenite grain size.
- Low-temperature inter-granular fracture – It is a mode of fracture which occurs along grain boundaries because of the micro-segregation or precipitation of second phases along grain boundaries. In low alloy steels manganese sulphide precipitation is frequently found as a result of high temperature treatments normally above recrystallization temperature of steels (around 1,250 deg C). These particles act as void nucleation sites for inter-granular dimpled fracture.
- Dimple rupture – It is a type of fracture where voids nucleate at inclusions and fracture occurs when these voids grow and coalesce under straining conditions (frequently referred as ‘void coalescence’). Manganese sulphide is known to decohese from the matrix even before straining, while most oxide inclusions decohese at small strains. This is related to the cohesion bonding by thermal expansion coefficient. The resulting surface is a relatively equiaxed dimple fracture surface.
The first two modes normally occur below the ductile to brittle transition temperature, whereas the third occurs above the transition temperature. Figure 5 shows the nucleation of voids at small strains (5a), large strains (5b), and fracture of steel (5c).
Figure 5 Nucleation of voids at small strains, large strains, and fracture of steel
Ductile and brittle fractures are the two main types of failure in low alloy steels. Ductile fracture occurs when the material is exposed to high temperatures while brittle fracture occurs normally at low temperatures. Fig 6 shows the difference between static and dynamic fracture mode curves, characterized by the differences in the strain rate applied. There are two tests to evaluate static and dynamic fracture modes. The Charpy V notch test is used to assess dynamic fracture and the ‘crack tip opening displacement’ (CTOD) test to assess quasi-static fracture toughness. CTOD testing is applied to materials which can present some plastic deformation before failure of a component. The measurement of this displacement is very important for engineering purposes and the importance of this test relies on the accurate measurement of this parameter.
Fig 6 Schematic showing relationship between static and dynamic fracture toughness
Another important factor which affects toughness is the anisotropy in fracture behaviour of hot rolled products. This is associated with the orientation of elongated inclusions or inclusion clusters. The highest energy absorbed occurs in the samples where the crack plane is normal to the elongated inclusions, and the crack can be deflected along the interfaces of the inclusions. Lower energies are absorbed when a crack propagates along the interfaces of the elongated inclusions. In Fig 7 two steels are compared, to the left conventional rolled steel can be seen, the anisotropy is higher due to the elongation of inclusions parallel to the rolling direction. If the material is loaded in this direction (red arrows) the strength is higher than if the material is loaded in the transverse direction (yellow arrows). In the steel on the right is steel with inclusion control. In this steel, the anisotropy is less due to better inclusion control which produces fewer, isolated and smaller inclusions. If the material is loaded in this case there is not much difference between the most and least favourable loading conditions.
Fig 7 Anisotropy of conventional steel and steel with inclusion control
Manganese sulphide inclusions are a major cause of fracture anisotropy. Due to manganese sulphide inclusions, transverse and through thickness orientations are the most affected by inclusion anisotropy. This can be improved by modifying sulphur containing inclusions to form hard inclusions which remain spherical during working or if the added cost is justified, the sulphur content can be reduced by further desulphurization or vacuum stirring.
Oxide inclusions are associated with ductile fracture which is characterized by linking of dimples. Void formation around oxide inclusions plays a dominant role in shear fracture. With increasing strength levels of the steel, the effect of inclusions especially at low temperatures is highly noticed.
Normally low inclusion levels are enough to guarantee acceptable ductility and toughness criteria in ultra-high strength steels. However, if in the future the demand to develop these properties, the need to avoid elongated manganese sulphide and minimize the number of coarse titanium nitride is to be pursued, as these are the most deteriorating inclusion types for ductility of relatively low impurity levels in ultra high strength steels.
Effect on fatigue – When the failure of a steel component has been due to a repeated number of load applications (cycles) below the yield stress of the material, it is considered a fatigue failure. In this regard, there are very important aspects in which inclusions play a major role. One of the first studies to establish a relationship between hardness and fatigue limit was the one carried out by Garwood and co-workers. Since then, the relevance that non-metallic inclusions have with regard especially to high strength steels has been the subject of several studies. Majority of these studies have pointed out several factors which relate to stress concentration, namely inclusion shape, adhesion of inclusion to the matrix, elastic constants of inclusions, and matrix and inclusion size.
Murakami and Endo developed the area model for evaluating the effect of small defects (holes) in metallic materials. In their study, they demonstrated that the problem of a small defect is essentially a small crack problem and hence this problem is to be solved with stress intensity factors instead of stress concentration. They found that there is a strong correlation of the maximum stress intensity factor with the projected area of the defect in a plane perpendicular to the maximum principal stress. Inclusions in fact can be treated as mechanically equivalent to small defects having the same value of the projected area (square root of crack area).
It is not only those factors which influence the fatigue life of a component, but also the location of the inclusion or defect inside the component. An inclusion of a certain size found close to the surface has a higher impact on the fatigue life than an inclusion of the same size in a location more distant from the surface. Murakami published quantitative equations for the prediction of the fatigue strength of a material with a surface defect, near the surface and an internal defect. Fig 8 gives classification of inclusion by location.
Fig 8 Classification of inclusion by location
The relationship between the harmful effects of inclusions on fatigue life and inclusion size is shown in Fig 9. This figure can help to illustrate that large globular inclusions are most harmful because of their size, not because of their shape. Also, that calcium sulphides compared with oxides of an equal size are less harmful. Finally, that the titanium nitrides are the most harmful type of inclusions over an equal size range compared to other oxides or sulphides.
Fig 9 Comparison of harmfulness index and inclusion diameter
The importance of inclusions on the fatigue of steel has been long recognized. However, the effect of type, composition, shape, and size of inclusions on fatigue has been extensively studied without a firm conclusion. A recent study has shed new light on understanding the effects of inclusions on fatigue which is summarized here. The fatigue limit is correlated with the existence of non-propagating cracks. It is not related to crack initiation. The fatigue limit is a threshold stress for crack propagation and not the critical stress for crack initiation. Fatigue limit correlates with hardness, upto around 400 HV. In this region, ‘the fatigue limit is determined by a material property showing the average resistance to plastic deformation of the material’.
As one passes the 400 HV threshold, the ideal fatigue limit, associated with the material properties cannot be reached, in general, due to the presence of defects (such as inclusions). ‘Defects smaller than a critical size are non-damaging (not-detrimental) to fatigue strength and the critical size is smaller for materials having a higher static strength, so that a defect of a given size is more detrimental to high strength steels than to low strength metals’. It has been demonstrated with several experimental examples that since the fatigue limit is a stress at which crack propagation does not occur, small defects can and have cracks starting from them which can or cannot lead to fatigue, depending on size and stresses. It has been argued that, for this reason, when a crack originates at the inclusion-metal interface or through inclusion cracking, the stresses within the inclusion are relieved and the inclusion domain can be regarded as mechanically equivalent to a stress-free defect or pore. Hence, tessellated stresses, for example can be less important than previously thought.
Using this approach, it is possible to find adequate relationships to predict the fatigue limit of high strength steels, reconciling the endurance limit relationship with hardness by including a term related to the cross-section area transverse to the loading, occupied by inclusion. Depending on the loading, position of the inclusions can be important, and this is accounted for. Thus, for inclusions close to the surface in rotation-bending, an empirical relation between the endurance limit and hardness has been proposed.
The effects of these insights on bearing steels development (SAE 52100 or 100Cr6) have been very important. Hence, for example, the results of, where particular relevance has been ascribed to different inclusion compositions, can be reappraised. The results indicate much less importance of inclusion type when analyzed in accordance with Murakami’s formalism, as shown in Fig 9. According to Murakami’s results, the largest inclusion present in the stressed area is responsible for fatigue failure. With the high cleanness of these steels, the classical methods of inclusion evaluation and quantification have been quite ineffective in predicting fatigue behaviour and extreme value statistics has been presented as a solution. In this context, Murakami developed a method for extreme value inclusion quantification.
Later, an ASTM standard was developed, mostly with the bearing community in view. With this method, Murakami and co-workers have been able to predict fatigue properties based on extreme value statistics for inclusions. Also, they showed that, when the inclusion population and inclusion size become exceedingly small, as in extra-clean electron-beam (EB) melted steels microstructural heterogeneities (bainite areas) are larger than the inclusions and act as fatigue nuclei. The importance of inclusions in fatigue is still the subject of frequent discussion, particularly in what is termed very high cycle fatigue.
The developments led by Murakami and co-workers on the understanding of the importance of inclusions in fatigue of high strength steel also had a profound impact in the inclusion engineering of spring and valve steels .Summarizing, when considering the literature on crack origination and propagation in fatigue, a person is to consider size and volume fraction of inclusions. Crack origination can occur ‘in the matrix’ or related to second-phase particles, in special inclusions. It seems that for lower strength steels, a critical crack size larger than the larger inclusions is needed for fatigue to occur. Hence, inclusions play a less important role in low strength clean steels. On the other hand, in high strength steel inclusions can be sufficiently large and play an important role.
Inclusions-steel interface condition also plays a role in fatigue life. On the other hand, the inclusions-matrix interface strength has been considered an important factor in the microscopy phenomena involved in fatigue cracking. Spriestersbach and co-workers, for example, noted that ‘classical’ inclusions (oxides, complex oxides and sulphides) debond easily due to the low inclusions-steel interfacial strength.
Also, differences in CTEs can promote inclusions-matrix separation. Hence, classical inclusions can be considered to behave as holes, as proposed by Murakami. On the other hand, titanium nitride, for example, has a strong bond to the matrix and the Titanium nitride-steel interface shows no separation. When titanium nitride is subjected to high stresses it cracks, and the cracks propagate into the matrix. Hence, the correlation between titanium nitride size and the fatigue behaviour can be different from the one observed for ‘classical’ inclusions.
Effect on machinability – Machinability comprises a wide range of parameters, including chip formation, cutting tool wear, surface properties of the machined work piece and environmental factors. Machining can be mainly described as consisting of two processes, metal fracture and metal removal to produce a certain shape or drilled holes at specific locations on the work piece.
Some oxide inclusions can have a positive effect on the process of chip formation (which is dependent on the ability to create a fracture along the structure) but can have a negative effect on the cutting tool wear which can overcome the initial positive effect on chip formation. Manganese sulphide inclusions also have a beneficial effect on chip formation, and the beneficial effect of high sulphur content on free machining steels have long been reported, because manganese sulphide inclusions do not cause cutting tool wear to the same extent as oxides do. A thorough investigation of the effect of different inclusion types on different steel grades for different applications has indicated that different steel grades have various inclusions with very different characteristics. Hence, these characteristics are to be optimized for each group of steel grades in order to make improvements to the machinability of steel without considerably producing a reduction in their mechanical properties
Perhaps one of the properties most traditionally related to inclusions is machinability. The effects of sulphides are well known and the design of these inclusions for machinability has been quite successful. Computational thermodynamics has been used to design steels with good machinability by tailoring sulphides to substitute lead ‘metallic inclusions’. Lead added steel presents important health hazards during steelmaking and has considerable environmental impact. Presently the automotive industry is defining minimum sulphur content for non-resulphurized steels to improve their machinability. This has posed an interesting challenge to bar manufacturers who need to adjust their processes to prevent nozzle clogging by the use of calcium in presence of sulphur in the range of 0.02 %. Stringent process control is needed, in this case
Effect on the nucleation of ferrite – Inclusions can play an important role in phase transformation. They also play a critical role in the nucleation of fine acicular ferrite in weld metal. This microstructure is important to achieve satisfactory mechanical properties with low carbon compositions without hot / controlled working. Thus, weld metal composition is tailored to cause the precipitation of adequate nuclei for acicular ferrite. Some of the factors considered relevant for an inclusion to act as a nucleus for acicular ferrite are crystal structure, differences in CTE, and depletion in austenite-stabilizing elements such as manganese around the inclusions. The latter is the most favoured explanation, followed by stresses generated by CTE differences between inclusion and the austenite matrix. This gains special relevance with the prospect of thin slab casting, where the extent of hot / controlled working which can be performed to refine the austenite grain is very limited.
One of the first and clearer in situ observations of the nucleation process has been done by Sugiyama and Shigesato who discussed in detail their observations on the importance of manganese sulphide on ferrite nucleation. Li and co-workers have shown experimentally and using first principle calculations that zirconium and titanium oxides promote manganese depleted zones in the inclusion-matrix interface, favouring ferrite nucleation. Also, they have shown that manganese sulphide can nucleate on zirconium oxide. As a result, they have shown the beneficial effect of zirconium-titanium deoxidation in the micro-alloyed steels, promoting finer and more uniform dispersions of manganese sulphide and acicular ferrite microstructures. Grong and co-workers reviewed the possibilities of producing ‘dispersoids’, inclusions with a sufficiently fine size and compositions to affect nucleation in solidification as well as ferrite nucleation. In order to achieve this, these particles, however, are to be formed in a more complex way than just resulting from classical inclusion formation reactions.
Effect on surface finish – Though surface finish can be considered a machinability issue, the case of tool steels for plastic moulds, for example, presents extraordinary requirements. Studies have shown that both microstructure and cleanliness play an important role. Simple forms of cleanliness quantification, however, are not able, in general, to correlate with polishing quality. Inclusion type play a definite role in the process as in the case of ESR (electro slag remelting) of P20 steel, for example, it has been demonstrated that the typical desulphurization of ESR is deleterious for surface finish and inert atmosphere remelting is to be used to prevent desulphurization and ensure that sulphides cover the oxide inclusions allowing a good surface finish. The compared results of VAR (vacuum arc remelting) and the so-called PESR (ESR under inert gas) can be explained in the light of this observation.
Effect on corrosion
Two examples of the influence of inclusions on the corrosion performance of steels are the importance of inclusions on hydrogen related failures such as hydrogen-induced cracking (HIC) and on the formation of pits.
Inclusions and hydrogen related failures – The importance of inclusions as traps and nuclei for HIC has been recognized at least since the 1970s. The importance of the synergistic effect of segregation and inclusions, particularly manganese sulphide was soon also recognized. Nakai and co-workers observed that shape control of sulphides had a high influence on HIC. They showed that higher oxygen in steel with type I sulphides (which do not have high plasticity) has better resistance to HIC than aluminum killed steel with type II sulphides which elongate during rolling. However, they preferred either calcium or cerium sulphide modification in order to ensure good properties.
It has also been clear that simply reducing sulphur and controlling sulphide shape is not sufficient to ensure good HIC resistance since crack propagation is controlled by segregation. Hence, lower carbon and lower manganese steels have been developed, as well as accelerated cooling strategies to promote less segregation, particularly banding, and more uniform hardness in the microstructure.
The interaction of segregation and calcium modification has been demonstrated in a study which has shown that in large segregates normal calcium treatment can be ineffective to prevent the formation of manganese sulphide. Thus, very low sulphur and avoidance of manganese sulphide has become the rule to ensure good HIC resistance. However, inclusions continue to play an important role on HIC crack nucleation. In very clean steels, it has been shown that manganese sulphide promotes hydrogen cracking. When studying the resistance of API X120 micro-alloyed steel, Huang and co-workers have related steel cleanliness to reduced effect of hydrogen, regardless of the inclusion type. However, they did not provide information on sulphur content of their steel.
Jinand co-workers reported that in calcium treated API X100 steel having 50 ppm sulphur, oxides are detrimental to hydrogen resistance. Domizzi and co-workers have not able to correlate sulphide length or sulphur content to HIC resistance in steels with sulphur in the 50 ppm to 150 ppm range. They propose that sulphur content and inclusion size influence resistance to hydrogen. They indicate that a small number of very elongated inclusions can reduce the HIC resistance in the same way as a higher number of shorter particles. They also emphasize the relevance of banding, which in micro-alloyed steels is normally associated with higher manganese contents.
Banding was also shown to be critical to the hydrogen induced failure of AISI 4140 bolts in sub-sea applications subjected to cathodic protection. Du and co-workers have shown the beneficial effects of generating a fine dispersion of oxide and sulphide inclusions through zirconium-titanium deoxidation on HIC resistance, when compared to conventional aluminum deoxidation. In a recent review, Ohaeri and co-workers have confirmed that inclusions in general can be harmful to resistance to hydrogen degradation but confirmed that elongated inclusions apparently have a more negative effect. The importance of elongated sulphides on the extent of hydrogen blistering has also been demonstrated. Hence, inclusions shape, quantity, and type play an important role in hydrogen cracking.
The eventual clustering of inclusions, particularly regions of microstructural banding, has a synergistic effect in promoting issues associated with hydrogen and is to be carefully avoided. Additionally, it is to be noted that Murakami has demonstrated that hydrogen trapping at inclusions has a considerable effect on super long life fatigue phenomena. It is evident from the above discussion that inclusion engineering plays an increasingly important role in the design of hydrogen resistant steels
Pitting – Wranglen reported the importance of manganese sulphide as pitting initiation sites both in carbon and stainless steels. He proposed that in carbon steels, the attack starts in the matrix close to the sulphide inclusion, which is nobler than the matrix while in stainless steels, the attack starts at the sulphide inclusion proper. With the development of characterization techniques, Ryan and co-workers have measured the presence of a chromium depleted region surrounding sulphide inclusions in stainless steels and proposed that these to be the initiation sites. They have not, however, propose a mechanism for the formation of these regions nor have described the thermal history of their sample. Their results were contested by measurements performed by Meng and co-workers in various steels (including Ryan’s original sample).
The observations of Ryan resemble the composition profiles around chromium carbides in sensitized stainless steel. While there a clear explanation exists for the formation of chromium depleted regions in the matrix around carbides in sensitization, it is not the case for the matrix surrounding sulphide inclusions. More recently, Williams and co-workers have observed a layer of iron rich sulphide surrounding the sulphide inclusions in stainless steels. This layer preferentially dissolves and starts the pitting process. In their conclusions, they have suggested that inclusion engineering can be used to control the composition of the manganese-iron-chromium sulphides and prevent this from happening. Park and Kang recently reviewed the issue of inclusions in stainless steels. They discussed the process of solidification of the sulphides which can lead to the situation observed by Williams.
It seems clear that subtle chemical composition differences around inclusions can be of paramount importance for the pitting of stainless steel. Results presently available indicate that the composition variations caused during sulphide formation can play a very important role. Liu and co-workers have recently demonstrated the effect of alumina clusters on pitting of carbon steels. Ma and co-workers have shown the anisotropic behaviour of pitting associated to manganese sulphide inclusions. Hence, it is clear that inclusions, in particular sulphide inclusions, have a crucial role in pitting. Park and Kang have remarked that the presence of oxide inclusions can also play an important role in pitting of stainless steels.