CW Manufacturing Process
The mill is always assembled with some basic design concepts like single or multiple profiles, style i.e. manual or automatic, capacity (tons/day), speed (m/min) etc. The tool designer uses flower pattern design software to develop the successive stages of work and the design of the CW forming unit (for different profiles different sets of tooling is required).The tooling equipments and roll forming steel (ALSI-L6) or its equivalent steel is used to fabricate parts that have a greater strength-to-weight ratio.
The long lasting tools are made from High-C & High-Cr hardened tool steel. Often to cut down the machine cost simulated tooling are used for simple prototype profiles. The number of stations might be reduced with an increase in the horizontal center, distance between rolls etc. The rate of CW production depends greatly on the steel strip thickness, additive core density and the bend radius; it is also affected by the sheer number of stations used. The lubrication is necessary to reduce the friction; and avoid the heat build up which may reduce the roll gap (strip can be roll formed using a precision lubrication system in combination with a vanishing oil or low residue soap.).
Forming & Filling - Mild lubrication wiping on the strip is provided just before the strip entrance into the mill. After entrance, the strip moves into the rollers of the internal guide roll sets, which are arranged up & down adjacent to each other, to create tension on moving strip so that it does not shift from its central line.
As the steel strip is driven through the machine through a predetermined path of travel, its lateral profile edges are gradually transformed from flat ends into an unequal downward bend. Here the length of one edge is deliberately made longer than the other. On further movement the central flat portion of the strip between the two bent edges is pushed vertically down to acquire a U shaped profile, during this time both peripheral edges are pulled up till they are horizontal with respect to the axis line of draw. The edges are then knurled with the help of the knurling unit which is a prerequisite for firm gripping between the two peripheral edges during seam lock formation.
Between pre-forming and forming stations a provision for the powder filling station is made, for dosing a powder additive in the U shaped trough. The powder feeding device is connected with the overhead multi chambered automatic additive storage and distribution assembly or any simple filling system, which contains, a middle bucket, a feeding hopper with a watch window, a step motor drive and an adjustable feeding spout.
A step motor is used to drive the feeding belt and adjust the belt speed. While roll forming, the steel strip with a U form passes horizontally below the powder filling station and the channel is simultaneously filled with powder little over excess in quantity. The forming rolls in turn then removes the excess additive and converts the U shaped channel into an O shaped tube while encapsulating the core additive. The two unequal peripheral edges with this transformation come closer to each other vertically over the core(vertical cross section of CW at this stage look like a shadow of apple with a stalk). The seam lock formation starts with the bending of the longer edge onto the shorter edge and then the united strip joint is twisted to form the lock. The coil, after forming, enters into the finishing dies where the CW diameter is corrected by indenting an inverted V notch lapping on the sheath longitudinally almost at opposite sides of the seam lock causing a smooth, few mm in depth, horizontal indentation. This action brings the oversize coils to its desired diameter within a ±0.5mm tolerance. The CW delivered from the exit port of forming unit reaches the coiler drum through a guide for winding. After winding the CW coil is removed and it then goes for packing.
Quality of CW
Depending on the nature of treatment and quantity of melt, the types of CW are chosen by the users. Besides physical details (like coil OD/ID, length, diameter, weight per unit length of steel strip & additive name grade and packing details etc), the following technological features are desired in coils.
• Coil texture - free from surface defects, additive segregation, entanglement, twists, etc.
• Coil properties - such as conformity & uniformity in sheath thickness and coil diameter and chemical composition.
• Coil Core density - high & consistent weight per unit length.
• Seam lock - firm locking.
• Coil rigidity - firm against deformation during coiling & de-coiling and high speed penetration during LMT.
To build the above characteristics in CW for its entire length, the following adversities are often encountered. The CW jacket is susceptible to corrosion and mechanical damages, holding the CW under unfavorable environmental conditions or improper packing, may affect the surface quality. Inadequate particle size or less additive filling or both, may result in segregation of grains, twist, and entanglement of coil.
The steel strip & additives should be resourced from genuine and authentic producers. Uniformity in jacket thickness and cross sectional geometry is achieved by strict roll forming process control and tool setting. The CW when not uniformly filled with additive results in segregation of particles & it becomes susceptible to entanglement such imperfections can be avoided by uniform & compact filling of granules per unit length of coil, the packing density (g/m) and surface area (m2/g) of additives are controlled within the specified limits, which facilitate the dense packing of CW. An excess additive filling followed by removal of excess portion and compacting is necessary.
The firm seam locking is dependent on the formation of two peripheral strip edge widths, and their interlocking technique. Negative edge width may be harmful since it will reduce the interlock surface area. Insufficient edge knurling, improper lapping angle of the taller edge on the shorter edge during seam lock formation creates loose contacts between the strip edges and may also result in the opening up of seam lock
The rigidity of coil is dependent on thickness of the steel jacket, cold working, additive packing, melt fluidity, seam locking and injection speed. The low feeding rate increases the CW traveling time, and consequently could release additive. From the above comments on quality it is evident that the inspection & testing of CW is necessary to ensure a higher and consistent level of quality.
A discussion on quality of CW
The specification of CW projected by venders does not refer to any mandatory standard. Similarly buyer’s tender specifications lack the justification and reference of the authentic source of information.
The objective of this discussion is to highlight the present day need for a comprehensive universal specification on CW used for LMT. This statement is corroborated by the fact that; the steel strips after cold working and the additive process lose their identity after shipping and it becomes a new composite material inferring that the CW should be tested & certified for their resultant chemical & physical, application properties.
The test certificates of steel strips & additives offered by OMC are meant for CW producers and only useful for CW users as reference documents, but it can not be considered as a test certificate of CW. Some tests are often conducted on CW, but these tests are neither mandatory nor authentic. The guideline for evaluation of CW performance is yet to be framed by the users of CW. From above discussions it is evident that enough challenges are ahead which are to be addressed by the concerned authorities in near future.
Inspection & testing of CW
Although the CW coils differ in their chemistry, core density, weight, & diameter, it is difficult to differentiate them from each other, without a Universal Product Code (UPC) or traceability tags. The existing practice of CW inspection involves examination of physical condition of coil and collection of two samples one each from two ends of the coil (1m long) using a smooth cutter.
The samples are tested for gross & net weight per meter, core ratio (C/R) is calculated as additive weight divided by total weight per unit length of CW. The additive is chemically analyzed, rarely coil geometry and physical properties of the steel jacket are measured. In the absence of universal standard specifications, procurement buyers often specify the CW with the name of additive along with wire diameter, jacket thickness, powder density etc. The necessity of an authorized specification was realized by the Standardization Administration of PRC, and the above style of specifying CW was adopted. The CW specification was framed under the category of “Professional Standards” and prepared as part of their “ferrous metallurgy industry standard” for CW, YB/T053 during 1993 and revised successively during 2000 & 2007, given in Table 1.
Table 1: Ferrous Metallurgy Industry Standard, PRC (YB/T 053-2007)
From the table it may be seen that, in Chinese standard of CW YB/T 053-2007, they have included 14 different types of additives with their respective National (GB) or Professional (YB) standards.
From list of patents & bibliography it may be seen that in recent years the development work in this areas is becoming accelerated. The outline of a few of these developments are given below:
i) CW Injection process - In this work, improvement in yield of flux & alloying elements was established by controlling the zone of additives release, close to the bottom of the ladle, through optimization of steel strip thickness, grade, diameter of coil, bath temperature, and speed of injection etc. This concept has provided better additive- LM interaction, which is necessary for reproducible results.
ii) Strand cladding of calcium CW - In this work a CW is formed by gathering at least three strands of continuously fed elongated reactive metal wires into a bundle and aligning the bundle of wires with a continuously fed sheet of metal sheath. This process helped the quantity of calcium in the cored calcium wire increase per unit length.
iii) Automatic CW Coil winding - This is a design based on the image processing technique to meet the requirements of higher productivity (yet to get practiced in larger diameter CW).
iv) Roll forming & additive fill monitoring - When a steel jacket is cold-formed from strip, the yield strength, and ultimate strength, is increased, particularly in the bend area and the seam lock portion of CW becomes stronger than the rest, but inadequate roll pressure, incorrect bend angle or folding relationship between strip edges may result in poor gripping of the seam lock. To avoid this defect an electro optical inspection device has been developed for monitoring the seam lock formation, similarly a digital camera is used to monitor the additive filling rate in the U shaped channel to avoid insufficient filling, these developments are used as a tool for rejection control of the finished CW.
v) Tooling - The roll forming systems produce a single profile configuration at a time, however to produce different profiles out of the same machine different tooling cassettes are being developed for automatic & quick change over.
vi) Coil Packing -To protect the CW from damage, customized & sea worthy (conforming to salt spray fog test ASTM-B-117) corrosion protective plastic shrink materials are used for coil packing. When heat is applied with a heat gun or placing in a hot chamber plastic shrinks tightly covering the CW coil. The most effective corrosion protection packaging solution is to use are plastic materials containing properties of volatile corrosion inhibitor (VCI).
vii) Core density - The non contact on line, packing density recording device is being tried with a LASER beam.
These developments are certainly helping “CW mills, coils, injection machines & applications” become more efficient, versatile & economical. Researches on LM treatment of CW are also being undertaken at various institutions in India & abroad (e.g. CMRDI-Egypt, ATS Ukraine, HIT-Harbin China, IIT-K India, etc).
Conclusion & Recommendations
It is understandable that CW produced by modern technologies is superior in quality than those produced by the conventional methods and often this technological gap becomes the reason for performance variation of CW. It is obvious that the performance depends on the quality of CW, but it is also dependent on nature of feeding and process parameters maintained during the ladle treatment. For quality assessment of CW it has now become essential to invite the attention of BIS to initiate dialogue with the relevant agencies for creation of a standard specification CW, method of inspection, sampling and physical/chemical analyses etc. Once the CW specification is made, the standard operating practices can be formulated and implemented at plant level.
In this context it would be fair to mention that the application of CW has improved the quality of steel economically, but still the CW industries are required to improve their reliability, quality consistency, and technical services to achieve its desired level of performance efficiencies. For further reading some references of articles/reports pertaining to CW applications are attached at the end of this paper as bibliography. It is recommended that the standard specifications on CW and CW consumables inclusive of their inspection, sampling & testing procedures should be framed by the appropriate authorities, for sustainable growth of CW industries.
For framing this standard the existing Chinese standard on CW may be used as a reference document and necessary modifications may be made. Further ASTM A1025 - 05 “Standard Specification for Fe-alloys, General Requirements” & ASTM E32-86-2006 “Standard Practices for Sampling Ferroalloys and Steel Additives for Determination of Chemical Composition”, ASTM A 495-06 “Standard Specification for Calcium Silicon Alloys; Standard grade of CaSi, CaMnSi, CaBaSi & CaFeSi” and DIN-17580 CaSi Technical delivery conditions and EN-10204 for inspection certifications may be used by concerned authorities.