TECHNICAL ARTICLE:

Introduction

The leading issue in the field of hot rolled tube production is concerned with continuous rolling mills where mandrels are operating either in a free floating mode, or in a mode based on a partially retained datum speed movement. The operational conditions of heavy-duty mandrels demand a systematic method of material selection in addition to the increase of strength characteristics, such as surface hardness, which consequently ensures the hard-wearing properties able to cope with high contact loads and temperatures.

Steel alloyed by a range of materials – including chrome, vanadium, molybdenum and silicon - is used for the effective increase of mandrel operating life. On one hand the carbon content has a marked influence on the occurrence of cracks during the conditions of cycling temperatures, while on the other it can also increase the hardness of steel; 0.3 per cent of carbon content is a crucial value. A higher carbon content correlates with the increase in crack initiation.

The mandrels have to be thermally treated in order to obtain optimized mechanical properties. These mandrels are chrome-plated for the reduction of friction coefficient and energy-power parameters for tube rolling (at Dalmine SpA, Italy, for example). The plated chrome has a high hardness HRC up to 60-80. The technology of this chrome-plating was thus recommended for the ‘159-425’ mill at Volzhsky Tube plant, Russia.

Research Highlights ‘Defect Formation’ on Mandrels

The research work and analysis on mandrels’ operation resistance, delivered by foreign companies, has illustrated that wear exists and rough linear defects – with the depth up to 2.0cm – are considered to form the defective characteristic.

Such investigations have been conducted on polished specimens selected from operated mandrels. They have displayed that formation of these defects is due to a number of processes that arise on the mandrels’ surface during operation. These include the initiation and development of pitting corrosion on the surface-metal boundary (Fig.1), formation of cracks inside of the coating (due to different linear steel), and chrome expansion coefficients at changes in cycling temperature (heating/cooling).

Figure 1: Formation of pitting corrosion on the boundary of the mandrel's
surface metal and chrome coating (when 195mm mandrels/156 tubes have been rolled)

In conclusion, the metal thermal fatigue is derived as the result of thermal-cycling changes of temperatures and loads, thus leading to appearance of sign-variable tensions – affected by tension-and-compression. This in turn results in the occurrence and development of a network of cracks directly on the mandrels’ surface metal layer.

It is well-known that when chrome with more positive potential than its host metal (due to air passivation) is plated, then the pair of elements being formed at the partition boundary results in the corrosion of a less positive metal. Unfortunately, in practice, this situation does not have enough attention paid to its consequences.

Analyzing Corrosion Cause and Effect

Chrome coating heat treatment (undertaken at 200ºC), which is conducted for hydrogen removal, accelerates the corrosion formation. The mandrels’ surface heating during the operation process is capable of reaching much higher temperatures, a factor that leads to coating heating up to Ac3 temperatures and the metal surface up to Ac1. This temperature factor accelerates pitting corrosion development, which is also due to the permeation of lubricant and cooling medium via chrome cracks and pores. Corrosion develops more intensively in metal than in its coating, a damage that then occurs more intensively in the coating the worse the metal becomes (Fig.2).

Figure 2: Intensive corrosion development in the mandrel's metal

The provoking influence of corrosion processes, in conjunction with thermo-cycling loads, results in the initiation and development of fatigue cracks of two kinds (Fig.3). These are vertical or under a small angle to the surface and linear – formed either at the vertical crack direction changes or on some zone of the vertical crack, where the linear crack starts to grow (the right part).

Figure 3: The mandrel's surface after 1,053 tubes rolling. The vertical growing crack changes the direction and is growing in the linear direction (the right part of the fig). The linear crack is forming and growing deep down from the crack which is growing under a small angle to the surface (the left part of the fig).

Moreover the vertical crack progresses deep down (the left part). It may be assumed that linear cracks are appearing and growing inside of those metal volumes where the transfer from one strained state to another is taking place (i.e. from compression to tensile stress).

Figure 4: 269.5mm diameter mandrel's surface state after 1,393 tubes rolling

The splicing of vertical and linear cracks causes the metal and chrome particles to crumble (Fig.4), and carries them out on the mandrel’s surface. These particles are then carried away by the billet during the rolling process, and the traces remain in the form of marks (Fig.5).

Figure 5: The appearance of marks on the mandrels' surface after rolling

Figure 6: Burn-back network of cracks and the beginning of rough
defects appearance (in the shape of ‘comet tail’ and dent)

However the process of rough defect appearance builds up gradually as the network of fatigue cracks develops intensively (Fig.6). Therefore newer and newer micro-volumes become involved in this process. As the quantity of the rolled tubes increases, the marks become longer, wider and mutate into rough linear defects (Fig.7).

Figure 7: Rough defects on the mandrel’s surface after 875 tubes

The high initial chrome hardness (HRC 60-68) is abruptly reduced during the process of mandrels operation and heating of their surfaces up to Ac3 temperatures, approaching the hardness of a tempered surface layer. There is evidence of its positive effect only at the initial short period of mandrels service (influences on friction coefficient reduction). It influences further negatively, damaging operating mandrels' surface relief because it has the direct effect of the formation and development of microscopic pitting which is the formation basis of rough linear defects.

Chrome Coating Micro-Hardness

The data of changes in chrome coating micro-hardness is presented in Fig.8 (curve b). The micro-hardness depends upon the quantity of rolled tubes and the consequent heating cycles, the result of which heralds the most intensive coating hardness reduction at an initial period of the mandrels’ operation. It accounts for less than 10 per cent of service life.

Figure 8: Mandrel's metal surface micro-hardness changing under the chrome coating (a), chrome coating (b) and white layer(c) depending on the quantity of rolled tubes

At the same time the high contact pressures lead to surface heating of the last two temperatures close to Ac1 - even during the short period of contact time of billet and the tool (mandrels). As the result we have a high temperature structure tempering and hardness reduction up to HRC 24-30 (Fig.8 – curve a).

On Fig.9 the change of mandrel metal micro-hardness under the coating is given subject to the quantity of rolled tubes. It results from the fact that the bigger quantity of tubes being rolled, the deeper the metal surface heating and the tempered weakened layer.

Figure 9: Mandrel's metal micro-hardness changing under the chrome coating
depending on the quantity of rolled tubes (a1, a2, a3 – the depth of the weakened layer)

The weakening of metal micro-volumes leads to reduction of plastic deformation resistance first of all in those metal volumes where the carbon and alloyed elements content is reduced. This can be seen beginning from the billet as the result of segregation non-uniformity of chemical elements.

Thus chrome that has low heat conduction, and is being damaged at a fast rate, does not preserve the surface of the mandrels from intensive heating. Instead it leads to reduction of surface heat-resistance and weakening, reduction of hardness and plastic deformation resistance. In these layers the plastic deformation has the place in separate metal micro-volumes on the surface. The fatigue cracks are inclined to the direction of tube rolling, which is the evidence of plastic deformation of these layers (Fig.10).

Figure 10: Plastic deformation of metal surface layers to the tube rolling direction

Increasing Wear Resistance

Therefore the improvement of manufacturing technology for large-sized billets, permits heat treatment technology to obtain the deeper layer of optimum tempering structure (tempered martensite) with uniform high hardness on the surface. This is one of the ways for increasing of wear resistance.

As the lubricant plays the smallest role in the surface process taking place on the mandrels (the subject of separate discussion) it should be noted that it has to possess the low heat conductivity.

The study of relief for the working mandrels surface has shown that sections of a white non-pickled layer is formed in the places of chrome absence (Fig.11). It represents a martensite without structure, of high dispersible and hardness properties, followed by the transition to a slightly pickled layer that transfers to the unchanged layer of the main metal on the mandrel.

Figure 11: White layer on the mandrel's surface after rolling of 2,313 tubes

It can be assumed that the operational conditions – high surface heating temperature (due to high contact loads and friction), and heating at the expense of heat conduction (at tight contact with the hot billet) – then leads to post-cooling and white layer formation with rather high hardness (HRC up to 50-54). This does not depend upon the amount of rolled tubes, for example more than 2,300pcs (Fig.8-curve c).

Residual compression stresses arise on the white layer during its formation that prevents initiation and development of fatigue cracks. This is proven by their absence in white layer zones. As soon as the number of rolled tubes is increased the thickness of the white layer may become reduced due to its wear. However the operational conditions of the mandrels support the formation of new portions of this layer that are propagating deep into the metal.

Thus the white layer depth is changed very little, dependant on the quantity of rolled tubes, and can be determined by two processes – wear and new portions formation. The depth of the white layer influences greatly on the depth of compression stress development, which increases as soon as the depth of the last increase, and in turn influences positively upon the cyclic loads resistance. Arising residual compression stresses prevent formation and development of fatigue cracks at these zones.

Conclusion

As the process of initiation and development of fatigue cracks depends upon the stressed state of metal surface layers, the most favourable residual stress sheet - from the point of fatigue damage resistance - will be where compression stresses are transferred into tensile stresses at a large distance from the surface.

Compression stresses are increasing in the white layer and their propagation depth (as mentioned below) becomes bigger with the depth increasing of this layer that is then followed by a slightly pickled transition layer and the layer of the main metal. It thus creates difficulties for the initiation and development of fatigue cracks.

Thus the positive influence of the white layer has high hardness against cyclic damage, and high wearability that makes its production development very realistic and prospective. The following conclusion comes from the above mentioned. It is necessary to conduct the preliminary creation of a white layer on the mandrels' surface for their workability, increasingly taking into account that their operation conditions initiate the formation and support of necessary white layer depth, as opposed to ecologically unsafe and expensive chrome-plating.