Failure mechanisms of continuous casting rollers and possible remedies

July 21, 2011 6:53 am

“Cladding, by conventional welding, over inexpensive forged materials and with the development of New Generation clad materials can produce continuous casting rollers with outstanding properties and minimize roller degradation and its failures”, describes Sidheshwar  Ghosh, key industry manager (iron & steel business) at Welding Alloys South Asia (P) Ltd.
Continuous casting process is widely used casting technique in steel plants to convert liquid metal into slabs and billets etc, and this accounts 90 per cent of steel production globally. The supporting rollers are one of the main components in the continuous casting and their main function is to support and guide the red-hot billet or slab and allow further solidification (Fig 1). Continuous casting rollers during operation are subject to especially harsh environment (Fig 1b). High slab contact pressure, elevated temperature, plastic deformation of roll surfaces at contact areas, sever abrasion by oxides and slags, cyclic thermal and mechanical stresses and above all corrosion either alone or in combination, contribute towards the deterioration of caster rolls. This aggressive environment together with the demand of increased output under high quality finished product, is forcing this conservative industry to develop new materials for rolls. In addition, considerably more attention is now being paid to the role of corrosion, oxidation and thermal fatigue due to the complexity of these phenomena in continuous casting conditions.
The development work of cladding rollers by welding has seen major improvements in the last two decades, which in turn has led to a substantial increase in steel production. This trend is expected to continue for the foreseeable future spurred by the development of new clad materials, which have already shown dramatic improvements in the service life of rolls in specific applications. Cladding by conventional welding, with aid of inexpensive core materials and with the noble metals can produce continuous casting rollers with outstanding properties.
Roll failure mechanismsIn general, there are two types of roll failure mechanisms: one is catastrophic failure, such as neck breakages, cracks through the body and spalling and second one is surface deterioration, where surface degradation or loss of size from whatever causes forces the removal of roll from service.
Catastrophic failureA considerable number of conventional cast rolls have surface wear resistance sacrificed to ensure that sufficient core ductility and toughness remains, thereby avoiding the danger of catastrophic roll failures. Figure 2a shows the roll with thermal fatigue cracks and these cracks will propagate through the cross section. The roll then breaks into two parts as shown in figure 2b, if the roller has not sufficient toughness to resist the crack propagation. The catastrophic failure occurrence with tough core clad rolls should be much lower and rarely happened than the conventional rollers. Rolls with a clad outer surface are usually produced from a tough and ductile forged core on to which minimal layers of the cladding are applied. These core material are therefore capable of being subjected to considerably higher mechanical stresses than cast rolls of either iron and steel and thus the most economical materials can be chosen as long as the mechanical properties are adequate for the application considered.
Spalling of the surface may occur in virtually any method of roll production: however, under the new welding methods and controls allied to roll welding experience, the incidence of such defects is probably below that of rolls produced by other methods. Thousands of rolls in service with necks repaired by welding are testimony to the acceptability and reliability of this roll repair method. The incidence of neck breakages of correctly welded rolls is no greater than breakage of rolls with no welds.
It can be concluded that catastrophic failure occurrence generally should be lower with tough core clad rolls than by the conventional roll production methods.
Surface deteriorationThe causes of roll failures are due to a variety of mechanisms and would include high contact pressure, abrasion, thermal fatigue, crack propagation, oxidation and scaling plus corrosion in the presence of water and slag.
OxidationIt is known that iron above 600° C in the presence of air readily oxidises producing a scale. At lower temperatures the proportion of the oxygen to iron may vary, however, the oxide layer is readily formed. This friable layer is not particularly adherent to the substrate and removed by friction with material being processed. So, with every roll revolution a quantity of material is readily removed and can be the major component of wear.When chromium is added to iron, and provided it remains in solid solution and not as carbide or other precipitate, chromium oxidises at the alloy-oxide interface. Since Cr2O3 has little solubility in FeO, it remains as island of a spinal FeO. Cr2O3 in a matrix of FeO. When the chromium content is sufficiently high virtually a continuous layer of Cr2O3 forms at the alloy-oxide interface. At temperatures below 1,000° C this continuous layer is achieved with free Cr> 12 per cent. The homogenous Cr2O3 surface presents many important features for rolls, namely a lower friction coefficient and superior gouging resistance. It is self-healing i.e. if removed by severe abrasion it regeneration a new integral surface. It also shows a considerably reduced tendency to deteriorate by adhesion, and is more corrosion resistance than alloys not containing this element.
Mould Flux Induced CorrosionThe slab skin is covered by a mould flux which typically contains inorganic fluorides like calcium fluoride, calcium oxide, aluminium oxide, silica and sodium and potassium oxides and some carbonates. The presence of this flux generates a highly abrasive environment resulting in excessive wear on the rolls. More importantly, reactions between cooling water and mould flux by-products generate a very acidic and corrosive environment with an acidity of 3-4 pH and the intensity of this corrosion, further increases with temperature.
Top zone rolls which are placed immediately underneath the copper mould and are subjected to this type of corrosion. This is the main corrosion mechanism of the top zone rolls. The service life of these rolls mainly depends on the surface material and high corrosion resistance alloy could perform better for this corrosion.
Localised Corrosion or PittingPitting corrosion or localised corrosion is equally important as a cause of surface deterioration of rolls. Rolls are subjected to sprays of cooling water which always contain salts and levels of NaCl, for instance have been reported up to 700 mg/l. The creation of corrosion pits and crevices may take place due to various causes, and the main mechanism is formulated below:2Fe + 3/2O2 + 6Cl- + 3H2O = 2FeCl3 + 6OH-2FeCl3 + 3H2O =Fe2O3 + 6HCl
As a result of the above chemical reactions the pH of the solution in the environment can be as low as 3 creating a very acidic and aggressive environment leading to easy initiation of pitting especially in sensitised regions i.e. Cr depleted areas and reheated zones, as shown in Fig 3. In addition, cyclic mechanical stresses and thermal expansion stress damage the passivating surface allowing corrosion to proceed.
Localised corrosion resistance relies on the stability of the passive film and is usually assessed by the pitting index which includes three alloys additions Pitting Index (P.I) or Pitting Resistant Equivalent Number (PREN): %Cr + 3.3 %Mo + 16N
The higher the PREN value the better the pitting resistance. The effect of those alloying elements on the polarisation behaviour of stainless steel is determined by the alloying addition’s influence on the stability of the oxide film. It is important to emphasise that the beneficial effect of those alloying elements can be felt only when they are in the solid solution form and not tied up in the form of precipitates.
Stress Corrosion CrackingA significant portion of rolls is subject to corrosion, as discussed above, and tensile stress, which are two necessary factors for stress corrosion cracking. SCC is believed to be nucleated at pitting damage sites and develops under the action of local tensile stresses as a highly branched network of fine cracks. At each crack tip the combined action of the tensile stress and specific ions in the corrosive media cause continual crack propagation with little evidence of local deformation. According to the current most plausible model, a cycle of events probably takes place as described below and illustrated in Fig 4.
1. Metal at a crack tip or imperfection corrodes until passivation is complete2. Creep continuous in the metal at the crack tip, plus induced stresses (mechanical and thermal cycling), increasing elastic strain in the film3. At a critical strain the film ruptures, the crack extends and the cycle is repeated.
Crack propagation in the case of Cr-C containing materials is often as a result of active path mechanism. Precipitation of chromium rich carbides along the grain boundaries weaken the grain boundaries and the cracks tend to propagate along the boundaries as illustrated in Fig 4b. A solution to this problem is to increase the resistance of the material to localised corrosion and to improve the repassivation kinetics of the oxide film, so that once the oxide film is ruptured, then it becomes easier for the film to revoke itself and form a protective film at the crack tip to delay the crack propagation. It is well known in stainless steels that nitrogen is the most effective alloying addition to increase the repassivation kinetics, especially when it is combined with molybdenum.
Corrosion fatigueCorrosion fatigue depends on the combined action of cyclic stresses and a chemically reactive environment. Self-sustained crack growth under corrosion fatigue is controlled by the response of the localized microstructure ahead of the crack tip under cyclic stress, although there is no clear dividing line between stress corrosion cracking and corrosion fatigue, it is believed that corrosion fatigue tends to take place at wider material environment combinations than stress corrosion cracking, as cyclic stress can readily maintain a sharp crack.
Thermal fatigueThermal fatigue is one of the most common surface deterioration mechanisms and widely experienced in continuous casting rolls. Thermal fatigue is best described as the gradual deterioration and eventual cracking of a material by alternate heating and cooling in which free thermal expansion is partially or completely constrained. Resistance to thermal shock can be simplified by the following equation: Thermal fatigue resistance,           kσR =  –––––––           EαWhere k is the thermal conductivity, E is young’s modulus and α is thermal coefficient of expansion and σ is the yield strength of the material at particular fluctuating temperature. Thermal expansion and thermal conductivity are physical parameters and should be discussed separately from the major metallurgical significant factor α which is a cyclic yield stress. It is clear from the above equation that the materials with high cyclic yield stress (high resistance to ageing, high microstructural stability) and high thermal conductivity and low thermal expansion display better thermal fatigue resistance.
The initiation process relies on the rupture of the surface film and subsequent accumulation of localised plastic deformation. It is likely that the maximum stress will be localized in these particular areas the stress will be much higher than the bulk of the section. Crack initiation takes place in regions where microstructure is weakened due to solid-state reactions such as carbide formation, Cr depletion and dislocation pileup.
A solution to this problem is well represented and significant improvements have been achieved using Nitrogen bearing materials which are much less prone to Cr depletion and display excellent high temperature stability and greater resistance to overaging.
Abrasion, Adhesive WearThere is a mixture of rolling and sliding friction at the contact point with billet or slab. These frictions cause roller abrasion. For instance, mill scale on the slab which contains oxides, slag particles, causes degradation on the surface by abrasion. When a third party in not present, then severe metal to metal wear takes place between continuous casting roller and the slab. The quality of the oxide film on the roll materials is very critical and its brittleness and the stability are determining the roller life. The failure mechanism of the rollers in continuous casting is more complex and is mainly depends on the zones. The following discussions is aimed at emphasising the role of different failure mechanisms operating in different zones of the continuous casting line depending on the environmental conditions and will address the choice of an ideal material. The predominant failure mechanisms in different zones are summarised in Table 1.
Possible remediesDue to severe environmental conditions and complex interactive failure mechanisms influences on the rollers, expectations from ideal roll materials are very high and are summarised below:
• Maximum tempering resistance• High hardness, good abrasion and adhesion resistance• Resistance to thermal and thermo-mechanical failure• High thermal conductivity• Low thermal expansion• Good thermal cycling fatigue resistance• High temperature oxidation resistance• Resistance to pitting and corrosion• Maximum resistance to stress corrosion and corrosion fatigue• Good weldability• No post-weld heat treatment• Reasonably cheap
Rolls used in steel continuous casting lines have been found to display extended life expectancy if clad, by welding, with a surface simultaneously resistant to the contact with the hot slab. Modified 12 per cent Cr martensite stainless steels strengthened by carbon and with the addition of Ni, Mo, W, V and Nb have significantly improved the service life of low Cr-Mo alloy forgings (i.e. 13CrMo4.4, 21CrMoV5.11, 16CrMo4.4) and they have been used worldwide since early 1980s. The life of the rollers with conventional clad materials is unsatisfactory due to high demand in production rate. The mechanism of early corrosion initiation and cracking is well established and published elsewhere and this common problem is now overcome by applying unique alloys containing nitrogen bearing materials.
Choice of optimum cladding material in continuous casting lineTop zone or O rollsThese rolls are placed immediately underneath the copper mould and are subject to very high temperature oxidation and corrosion as well as some degree of abrasion. The main failure mechanism of the roll surface is believed to be general high temperature oxidation, corrosion particularly mould induced flux corrosion. The roll gap tolerance tends to be very critical in this zone as the Ferro-Static liquid pressure is the highest as the slab exits from the copper mould. It is therefore essential to maintain the roll gap in order to minimise the slab bulging and to reduce the likelihood of breakouts and there has been extensive development to improve the roll life. These rolls are 140 -160 mm in diameter and spaced along the length of the roll and typical life of these rolls is 40 – 80 melts. However, if the continuous casting line is not shut down regularly for other reasons it is essential to improve the life of these rolls. Recent work has confirmed that application of special nickel alloy (WA-Top zone-G) and super austenitic stainless steel (WA-Corresist) as well as nitrogen bearing 414 and DN alloys performed excellent results and improved the life of the top zone rolls significantly and is around 4-6 times. The open arc welding technique is particularly useful to clad top zone rolls since low heat input can readily be maintained and the adverse effect of excessive heat is avoided, unlike the submerged arc welding process.
Spray chamber rollsIn this zone a continuous supply of cooling water causes severe thermal fatigue as well as stress corrosion cracking leading to the formation of cracking on the roll surface. As discussed elsewhere, the commonly applied 12Cr-C and 12Cr-Ni-C cladding materials suffer from a lack of corrosion resistance (Fig 6a) and are now replaced by nitrogen bearing materials, for example WA-414N or WA-DN, which perform outstanding corrosion and oxidation resistance as well as wear resistance properties (Fig 6b). This is because of their high temperature microstructure stability and enhanced tempering resistance and higher resistance to thermal and thermo-mechanical fatigue. These materials have high toughness and better low and high temperature strength compares to Cr-C containing materials.
In summary, nitrogen bearing materials offer excellent properties and increase the life expectancy significantly as confirmed by worldwide experience. The salient features of these nitrogen materials are listed below:• Increases the stability and passivity range of oxide film• Increase the repassivation kinetics• Forms homogenous and finely distributed nitrides inhibiting the grain growth• Coarsening rate of nitrides is significantly lower
Due to the above subsequent benefits of the nitrogen bearing materials are:• Improved microstructural stability• Enhanced tempering resistance• Better low and high temperature strength• Higher impact strength and toughness• Improved oxidation and corrosion resistance• Higher resistance to thermal and thermo-mechanical fatigue• Improved resistance to fire cracking
Straightening zoneIn addition to water corrosion, the rolls in this area are subjected to higher stresses and consequently the high degree of stress corrosion cracking. These rolls also more prone to thermal fatigue cracks as well as thermo-mechanical cracking. Nitrogen bearing alloys (example 414N and DN) materials are once again the ideal choice in this zone because of their high temperature microstructure stability and enhanced tempering resistance and higher resistance to thermal fatigue.
Roll / Slab interface at bearing areaThe bending stresses are significantly reduced by the introducing split rolls in this area but an increase in wear at roll/slab interface is noticed. This is due to higher surface temperature and higher compressive stresses in these regions. Adhesion of surface scale on the roll surface under higher stresses also leads to adhesion and surface roughness.
Withdrawal zoneIn this zone the rolls are not subjected to a particularly harsh environment and life expectancy is very high and often cladding of special materials is not required towards the end of the line.
Cladding and refurbishment of rollsDifferent welding processes are applied for cladding of continuous casting rollers. The most common cladding methods in use today are open arc and submerged cladding. Because of its higher deposition rate, submerged arc cladding is often used. The open arc cladding is specially suitable for continuous caster lines and particularly advantageous on rolls below 250 mm diameter. Since the production of equivalent solid wire electrodes is quite difficult because of their hardness, cored wire electrodes have been successfully produced for a long time.