Rajiv Deshmukh, Technical Director,
Mehta Sanghvi & Co
Plate edge beveling is one of the very important processes in the metal fabrication industry. For a better quality fabrication, welding plays vital role and the correct and accurate beveling is the first step towards obtaining the best welds. We are going to study various beveling process options and their comparative advantages and disadvantages in this paper.
There are various process options for Bevelling such as,
- Oxy Fuel bevelling.
The first process is thermal process and usually creates Heat Affected Zone (HAZ). Therefore, depending on weld joint serviceability and the material composition, it is cautiously used. However, it’s use is now tapering down due to other factors as productivity, limited accuracy, and higher indirect costs.
Most commonly used Bevelling options are shearing and Milling processes (Non Thermal processes).
Basically, three types of bevellers are available. Hand held, Table top models and automatic ones.
For hand held and bench mounted types, milling process is the most preferred one as it offers ease of operation and economy. These are good for profile bevels, smaller jobs and radius bevels too. These come handy for small size pipe bevels also.
For large plate bevels, shearing and Milling both options are used. However, the latest trend is to use milling process as it offers distinct cost benefits, gives best smooth bevelled surface without any marks, dead accurate bevel angles and consistent root face besides big range of continuously variable bevel angles.
The milling type bevellers are preferred since these can be used on practically all the materials including weldox/hardox, SS, low and high alloys including super alloys like inconel, hastalloy and even armoured steels, aluminium and range of carbon steels..
However, milling and shearing processes have their own advantages and limitations. These are to be considered based on the mechanical properties of the material to be bevelled, the bevel angles needed and the bevelling environment. Also, the Bevelling speeds based on the material composition.
The economic considerations in terms of cost of the tool plays important role in the selection of either of these processes.
Milling inserts are quite effective Bevelling tool at a reasonable price. In this type only inserts need to be changed which works out to be quite economical. However, in the shearing type of machines, the complete cutter needs to be changed which works out to be very expensive as far as running cost is concerned; additionally inventory cost also goes up considerably.
The automatic cold Bevelling process offers some very important benefits, Viz,
A) Elimination of HAZ: Ensures the weld joint strength since the micro structure remains unchanged around beveled edge. The cold beveling eleminates the heat input andconsequently thermal stresses and improves the weldability.
B) Accurate bevel angles:Ensures optimum weld metal deposition. Reduced heat input, which ensures minimum distortion. Further, it saves on weld metal deposition as every degree increase in the bevel angle, increases the molten metal deposition by about 12-20%. This not only increase the direct costs for welding consumables but also adds up indirect fixed and variable costs like power, manpower etc..
C) Accurate and consistent root face over the entire length of bevel. This ensures the correct fit-ups most important for automated welding which needs to have consistent root gap and no mismatch. The success of welding automation largely depends upon the joint fit-ups.
D) Smooth bevelled surface.This avoids possible contamination on the surface to be fused, thereby avoiding the foreign particle entrapment causing the typical weld defects. This also ensures uniform and homogeneous deposition giving good side wall fusion and eliminating the hair line cracks and allied defects.
E) Minimum operator skill. These automatic Bevelling Machines are very easy to operate and eliminates human skills. It also reduces skilled manpower cost.
However, speed and finish plays important role in making a choice of shearing OR milling process.
One can get higher speeds by using shearing process but the costs are very high. Shearing is generally used effectively for plate thicknesses up to max 40 mm in multiple passes. Beyond these thicknesses milling certainly is a cost effective method. Besides, generally it can be used on almost all metals as seen in the foregoing paragraph.
A typical example based on computation of all direct working costs like tool cost and power cost on a specific type of materials for milling process shows the approximate Bevelling cost per meter is about Rs15 to Rs. 25 per meter going from low tensile to high tensile materials. Higher the tensile, higher the cost. Also higher the thickness, higher the cost but comparatively less than shearing process.
This is the reason that for higher thicknesses milling type bevellers are better; offering benefits of optimum speed and cost.
The shearing machines also run a risk of higher maintenance cost due to the possibility of breaking of cutter or else of bevel gear/gear box.
Compared to the milling machines the shearing machine cost per meter of bevel will be about 15% higher.
Above guidelines are to be kept in mind for the selecting the correct beveller for your specific job. The important factors are – Quality, speed, operating cost, maintenance cost and tensile strength of material and lastly variety of bevel angles involved.
In conclusion, milling machines are more cost effective over wider range of materials and thicknesses as against shearing machines which work betterin the limited range of thicknesses and are generally good up to mid level tensile strength plates.
Few points of comparison between Thermal Bevelling and cold mechanized Bevelling using milling process.
a) Thermal Process (oxy fuel cutting)
This process is not suitable for special alloys, high tensile steels and other thermal sensitive materials, as it will create a HAZ which changes the grain structure. Therefore, applying thermal Bevelling has to be done very carefully and selectively.
b) Mechanized Bevelling:The automated cold Bevelling process by milling process is suitable for almost all materials including high tensile steels, super alloys, titanium and other sensitive materials as well. It does not alter metallurgy of the parent material and therefore it’s very safe.
2) Range of bevel angles
a) Thermal Process: It is best suited for angels up to 45°. In angels >45° there are always chances of reduction in accuracy and hence the total included angle may increase resulting in extra weld material in the joint, further resulting in extra welding consumable, power, manpower and other costs like grinding wheels, gases etc. Also, increase in time and therefore increased throughput cycle time.
b) Mechanized Bevelling: This process is suitable from 0°up to 80° and in some cases more. Therefore, it can cover practically entire range of recommended Bevelling angels. Automated Bevelling using Milling process can be used on a wide range of plates starting from 5 mm to 200 mm, although the general range is 8 to 100 mm. By extending the bevel angle range up to 90° the automatic beveling by milling process offers the decladding option which can be very useful for large and thick cladded plates, used in heavy fabrication for critical and super critical applications such as reactor vessels, heavy machine building, super critical thermal power and atomic power stations, armoured vehicles etc, etc..
It offers very accurate bevel angels, consistent root face (land) and mirror finished smooth bevelled surface. Therefore, the joint fit ups are exceedingly good, avoid gaps and mismatches and such fit ups are best for automated welding. The weld quality and aesthetics is best.
Extra and unwarranted costs of welding consumables and other costs are eliminated thereby making it cost effective and fast process, not to forget very clean process as well.
3) Bevel Speed
a) Thermal Process: Speeds are limited as any increase in speeds beyond a recommended speed will affect the quality of bevel. It will also need extra heat input which will be detrimental to the plate being beveled as it will create bigger HAZ.
b) Mechanized Bevelling: This is high speed bevel process. This does not get restricted by manual operator skills or any other factors. The bevel speed remains consistent and can be adjusted for optimum output depending on the material being bevelled. Usually, max speeds up to 2.6 to 3m / min can be obtained depending upon the bevel process chosen.
4) Cost of Bevelling
a) Thermal Bevelling: Basic cutting cost constitutes of manpower, gases, grinding wheels, cutting nozzles, grinding, two operators (one each for grinding and cutting), accuracy of angle, indirect costs of extra weldment, extra consumables, gas etc.
b) Mechanized Bevelling: Direct costs of inserts and nominal cost of power are involved. Most indirect costs due to factors as explained in thermal Bevelling are avoided. Therefore, it’s an economical process. The cutting costs by this method compared to thermal cutting are lower by about 15-40% depending on various operating factors already explained.
a)Thermal Process: Considering the factors stated above, the productivity of thermal Bevelling is lower compared to mechanized Bevelling by about 20-45% based on all the operating factors, obviously therefore the mechanized Bevelling is a preferred option.
In conclusion we can say that cold mechanized Bevelling is faster, cleaner, reliable and easy to handle process as compared to thermal Bevelling.
Cold Mechanized Bevelling therefore is considered most acceptable process for ensuring higher productivity, improved quality at an optimum cost. In cold beveling processes shearing and milling are the options and milling is a better option for quality, workability over a wide range of materials, higher thicknesses, wider choice of continuously variable bevel angles (0°-80°) and choice of beveling for all types of bevel preps including J bevels..