CASE STUDY FOR THE WELDING OF REPLACEMENT KILN SECTIONS

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The Cement Institute

Cement machinery such as kilns and mills are exposed to dynamic loads. That indicates that when they are working, they are subject to fluctuating stresses as bending, torsion, tensile compression or combination of these.

Stresses like these can cause cracks and fractures formation without any kind of deformation and really overload, which means that the maximum tension during alternating loading has been well below the static tensile strength of the material.

This type of crack formation is identified as fatigue and most cracks and fractures in the components, in general, are caused by fatigue, in the same way, there may be deformations in the kiln due to an overtemperature in the operation of the kiln. In this case study, the various methods for the replacement of the kiln shell will be covered by having cracks and deformation.

WELDING PROCESS OPTIONS

Welding of the kiln installation or replacement sections can be done using three types of processes:

1) Submerged arc (sub-arc)
2) Metal inert gas (MIG)
3) Manual metal arch (MMA)

1) The submerged arc is a fully automated process where the continuous welding wire is submerged in a flux bed. Precise welding is carried out using a boom, which is placed on top of the kiln shell to maintain a flat welding position. The auxiliary motor or locking gears are used to rotate the kiln at the correct speed.
The welding deposit is very uniform and reduces the concern of the operator error. Being fully automated, the configuration is critical, and the process is suitable for the manufacture of the kiln section and, in some cases, for the replacement of the shell on the site. This process offers very high deposition rates.

2) Metallic inert gas (MIG) is a semi-automatic process that uses a solid or core continuous wire. A shielding gas protects the welding pool. Deposition rates are high, and the process is frequently used to fill and cover a manually welded root (MMA). MIG can be prone to lack of fusion of the sidewall and cold lapping, especially in the immersion transfer mode. This has been usually due to the variation of the parameters since the precise balance between volts, amps, and inductance must be achieved. This process is only suitable in the hands of competent MIG process operators.

3) Manual metal arc (MMA) remains the most commonly used process for kiln section replacements. The welding is completed with conventional electrodes covered with flux, being completely manual, the process is more versatile. There is little chance of depletion or lack of fusion of the side walls due to cold lapping with manual electrodes. The process is suitable for open site operation. Welding quality depends exclusively on the operator/contractor, so it is essential to employ a recognized company with fully certified welders who have experience in this type of kiln repair. Deposition rates are lower than with the previous processes and welding can be done in all positions.

The dimensions of the kilns vary in length and diameter depending on the process and the capacity of the plants.

A wet kiln can measure anything up to 160 m long and 4.5 m in diameter, while a dry process oven could be 75 m long and 5.0 m in diameter.

Rotax Kiln

SECTION REPLACEMENT DETAIL

In general replacement, the sections of the shell can be any length and up to 20 m.

The weight varies from 10 to 75 tons depending on the thickness and length of the material. The thickness can vary from 25 mm between tires up to 100 mm in the kiln tyre.

TIME SCALES

The specialist-welding contractor normally expects to take approximately 12 days of continuous work per average section (including two complete joints). There is an additional consideration of the downtime for the removal and subsequent replacement of an internal refractory or work of wet process kiln chains. The contractor’s time scale is only a guide.
The overall duration could be affected by careful planning with the provision of delivery time to the welding contractor, since the availability of material for the required dimensions is rarely found outside the inventory of the steel manufacturer. In the short term, small plates may only be available, for example, 8 m. This would require two longitudinal submerged arc welds, while an advanced notification would allow the contractor to send the material to the steel workshop on a plate, for example, 16 m. This would imply only a welded joint.

LONGITUDINAL JOINTS

Care must be taken to ensure that the longitudinal joints between adjacent sections do not match, as this will result in high levels of tension in the joint between the two longitudinal joints and the circumferential joint. However, ideally, the longitudinal joints should be separated by 90 °. In practice this is not always possible and 45 ° should be the minimum.

TRANSPORTATION

All sections of the kiln should be fastened with spider supports 1/2 meter from each end and in the center to avoid losing their shape during transport. If a tire support section is included, a clamp should be placed under the centerline of the tyre.

CAUSES OF FAILURE

Replacement of the kiln section is necessary due to a combination or direct cause of the following:
A. Distortion
B. Wear
C. Fractures or cracks

A.WEAKENED BY DISTORTION

The constant high temperature at the firing end of the kiln causes refractory bricks to erode. Wear accelerates the “hot spots” of the outer shell creating a distortion with excessive local heat, thinning then takes place due to subsequent oxidation and scaling. Kiln shell constructed with a boilerplate (BS 1501 151 430) must not exceed 350 °C. The service life of the shell is dramatically reduced with higher temperatures. Sections of the shell that operate above 350 °C should use a creep-resistant chrome quality plate (15 Mo 3).

It is recommended to completely replace the damaged area with a new shell section. The alternative is to patch the affected area; however, it is difficult to recover circumferential integrity due to the contraction effects of welding.

The table below sets out the stress rupture limits of the chosen kiln base material. Weld repairs become more difficult if the kiln has been operating at more than 400 °C for prolonged periods and crystallization of the material has occurred.

STRESS RUPTURE LIMITS

The resulting ovality can restrict the retention of taper magnesium bricks, since they are keyed in position with steel plates to the exact dimensions. Filling the gaps behind the bricks with mortar can internally rectify this distortion; However, long-term success is limited.

B. WEAKENING FROM WEAR

Loss of coating material in the chain area of a wet kiln can cause thinning and wear due to constant impact and abrasion. Over time, this causes the section of the affected kiln to need to be replaced. The refractory wears and breaks exposing the inside of the shell to the same conditions. You can also suffer wear in a dry kiln if the shell material is affected by chemical and sulfur attacks.

C. FRACTURES OR CRACKS

Fractures are often caused as a result of the misalignment of the kiln or by a great ovality and can be a combination of the failures mentioned above.

Cracks usually manifest in areas of high stress, such as kiln section joints or previous minor repairs, etc.

BASE MATERIAL

The kiln shells are mainly made of low carbon steel. There are variations in the exact specification depending on the original manufacturer, however, the most common grade for kiln sections is BS 1501 151 430A, which is fully ultrasonically tested according to the latest BS EN 10160:1999 standard.

In some cases, at the firing end where temperatures exceed 350 °C, a type of section of steel, chrome, molybdenum steel is used. It has improved properties at elevated temperatures and often meets the specification of BS 1501 243 molybdenum 0.3% (DIN 15 Mo. 3).

The mechanical properties of these materials must be taken into account, since it is important that qualified welding consumables with superior characteristics are recommended and applied.

ELECTRODE RECOMMENDATION

The high load and the thermal cycle supported at the high temperature of the kiln operation up to zero during the shutdown periods provide thermal stress to the welded joints. Therefore, it is important that a qualified electrode with superior mechanical properties be applied to the base material.

Low Carbon Steel Shell

A choice of two electrodes is recommended. Both conform to classification AWS E7018/BS 639 E5144 B26 and possess a fluorite, calcite and quartz flux.

EutecTrode® 7018RS
A basic single coated low hydrogen electrode with a thin flux covering which creates rapid solidification.

EutecTrode® 6666
Dual coated hydrogen-controlled electrode containing two separate layers of flux coating to provide improved arc stability and striking properties.
Current Settings (7018RS or 6666).

Type of Current
7018RS AC 70v min DC +
6666 AC 50v min DC +

The choice of electrode applied will d pend on the subcontract welder’s preference.

MECHANICAL PROPERTIES COMPARISON CHART

Chrome Molybdenum Steel Shell
This material is sometimes used at the firing end of a cement kiln. The mechanical properties of this steel provide superior creep resistance at high temperatures and pressures. When this material is incorporated into a cement kiln, a compatible electrode must be selected.

EVB Mo
A basic coated hydrogen-controlled electrode to classification DIN 8575 E Cr Mo IR25. Designed for 1.0 Cr 0.5 Mo creep resisting steels. Deposits weld metal of high· metallurgical and radiographic quality in all positions.

Type of Current

710 AC 70v min DC –

MECHANICAL PROPERTIES COMPARISON CHART

PRE-HEATING REQUIREMENTS

When welding thick and heavy sections, the importance of preheating is essential. This should never be taken as a general rule because the carbon level of the most common steels varies. The mass of the cross section determines the cooling rate and, therefore, the risk of cracking due to the formation of martensite, cementite and bainite. Therefore, it is a good practice to calculate the carbon equivalent and see the result of this figure in the preheating graph.

The degree of preheating is established by calculating the equivalent carbon value using a standard formula.

Considering the replacement section of low carbon steel according to specification BS 1501 151 430A.

Tensile 430 a 520 N/mm2

Composition:
C 0.22
Si 0.10
Mn 0.5 – 1.2
P 0.025
S 0.025
Cr 0.25
Mo 0.10
Ni 0.30
Cu 0.15

Carbon equivalent (%CE) = pre heat ratio

%CE = 0.52

Note: (see figure 1, which determines the preheating requirement from this calculation).

It can be seen that with a 30 mm thick low carbon steel cover, it would be advisable to preheat 200 °C at a distance of 150 mm on each side of the joint. This will reduce the risk of spreading cracks in the long and short term.

DIAGRAM TO DETERMINE THE PREHEATING TEMPERATURE OF CARBON STEELS AND LOW ALLOY STEELS

ELECTRODE QUANTITIES

Example:

Considering a shell replacement section 4.2 m diameter the material thickness between tyres is 30 mm (1.25″).

Trigonometry is applied to determine the cross-sectional area of the weld preparation.
Determine the length of ‘X’

X = 1.44″

Cross section area = X = ​​ 1.25″/2

Cross section area = 0.72 x 1.25 = 0.9 in²

Circumference
Dia 4.2 m = 165″
165 x 3.142 = 520″
Weld volume in³ 0.9 x 520 =468 in³

Total volume for both welds = 936 in³

Formula for converting to kg’s of electrodes : (50 in³ = 12.93 kg)

​​ 936 /50 ​​ x 12.93 = 242 kg + 20 % = 290 kg

20 % is allowed for stub end and spatter loss. This also allows for additional build up above the kiln shell surface.
The internal back gouging on this thickness of shell is likely to be 10 mm (0.4″).

BACK GOUGING

Cross section area = 0.08 in²

Weld volume = 41.6 in3 d

Total volume for both welds =83 in³

Applying formula: ​​ 83 / 50 ​​ x 12.93 = 21.5 kg + 20 % = 26 kg

Total electrode requirement = 316 kg

These works are only a guide. A variable material thickness can be applied. This equation applies to an electrode summary of any shell insertion section.

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