Lightweighting challenges for automotive steel tubes manufacturers
Lightweighting is among the most across-the-board trends in the automotive industry. It may involve replacing commonly used materials with lighter alternatives, manufacturing vehicle components with less input material, or it might even be a combination of the two. The end goals are raw material savings, fuel consumption reduction, and a lighter – one might well say – environmental footprint. All the while preserving safety and performance, of course.
Vehicles can be lightweighted, among others, with material substitution. It follows that precision steel tube manufacturers are also required to play their part for tube products used in cars and trucks like stabilizers, camshafts, propellers or driveshafts, steering shafts, door reinforcement, tubes for seat adjustments, and others.
Welding lines: upgrade existing or invest in new?
Many tube manufacturers have to face this challenge and either modernize an old proven tube welding line or consider buying a new welding line. Depending on the tube dimensions to be welded, this implies an investment sum from approx. 1.8 million euros up to 10 million euros for larger tube dimensions and extensive peripherals around the line. The delivery time, which can exceed 12 months, must also be taken into consideration. In addition, there is the training time for the operators, which begins only after the installation of the entire line. The choice of purchasing a new welding line needs careful consideration. All possibilities must be exhausted to prepare the old welding line for the requirements.
The modernization of an old system depends on the tube dimensions to be welded, the maintenance status of the system, and the single design of construction. Therefore, there are no ready-made solutions available. Nevertheless, some points that require maximum attention and consideration are the following:
1. Input material
Ordering the right bandwidth is critical: how is the tube-flower (material shape from one stand to the last stand) executed? Are the knife rollers at the right angle? Is there a strip edge trimming device included? I recommend the latter for larger wall thicknesses to eliminate possible inclusions in the strip edges and thus a partial uncertainty of the durability of the welding of the tube to eliminate. Unfortunately, as far as I know, there are currently no suitable strip trimming devices for 3.5+ mm high-strength steels on the international market. Tests have shown that existing ones are too weak. If the strip thickness range is aimed at 4.0+ mm, you should think about in-house construction or outsourcing the construction of a strip trimming device. The costs for pure machine construction with integration into the machine control of the entire system can easily exceed 250,000 euros.
Is the strip width constant over the length after slitting the coil, and are the strip edges free of damage during slitting (e.g. from broken knives and during transport and loading)?
2. Structure of the welding line
The number of stands in the forming or calibration area can only be increased with a disproportionate amount of effort, as this is associated with the lengthening of the machine bed. Therefore, a minimum number of stands should be available. This is less critical in the forming area but is essential in the calibration area. Especially when the ratio of diameter to the wall thickness of the tube to be welded is less than 5.5 and the inside diameter is less than 22 mm. In this case, the largest possible tube diameter should be selected at the welding table in order to get enough heat for the welding into the material with a sufficiently large impeder. In the subsequent calibration range, the outer diameter must be gradually reduced without damaging the tube’s surface. Since the reduction per stand is possible within narrow limits due to the material, a sufficient number of stands must be available. If a tighter range of the outer diameter tolerance is also desired, e.g. < 0.08 mm, hydraulic clamping of the pairs of rolls is recommended at least on the last two stands of the calibration.
3. Propulsion power and generator power
The material movement in the welding line requires a correspondingly high driving force. Ideally, the pairs of rollers at the top and bottom of a stand are driven individually. Are the shaft diameters big enough to transfer the forces that occur? Is the welding generator adequately designed and still reliable enough to weld high-strength material with the desired thickness? Does the working area of the generator correspond to the tube dimensions to be welded so that the capacitors do not have to be reconnected?
As a rule, smaller tubes should be welded at a higher frequency (higher power), and larger tubes at a lower frequency. Modern generators, which are designed precisely for the range of tube dimensions to be welded, get by with just one frequency range.
4. Inductors
Depending on the frequency, the tube to be welded, and the current micrograph of the weld, the zone affected by heat can be influenced to a certain extent with a coil of two or three turns. Generally, it is more a question of the manufacturer's welding philosophy. However, if the micrograph of the weld is borderline, or if the weld seam occasionally tears during the destructive tests, tests with coils with different turns can provide the decisive impetus.
5. ID scarfing
Regardless of hydraulic or mechanical scarfing, the use and location of the ferrite core with its coating are critical. Is the cooling selected in such a way that the epoxy resin jacket does not burn, and is there adequate heat to flow into the tube for sufficient welding of the strip edges?
6. Emulsion flow and cooling
The emulsion must be carefully used to cool and reduce friction between the material and the rolls. If used incorrectly, either the material can be deterred and cracks can arise or the tube may run too hot into the calibration area and the Turk's heads so that it cannot be permanently deformed.
7. Flying saw
It is also necessary to consider the higher strength of the tubes for the existing drive of the cutting saw. If this is the case, the saw is no longer the bottleneck with tubes made of high-strength steel, as it is with other high-speed tube materials. Because of this and the need to rework the tube ends, there is no need to invest in a quick cutter or so-called hot saw. On the market, there is a sufficient selection of suitable saw blades for cold cutters, preferably with a coating.
8. Quality
To ensure good performance, overall equipment effectiveness, and to keep the scrap rate low, downtimes must also be reduced, among other things. This is often at odds with the required assurance of quality, especially for the automotive industry. It is thus necessary to find a compromise between well-trained personnel, easy and safe access to the running tube on the welding line (especially in the finishing area), a conscientious and professional assessment of micro sections of the weld seam at a suitable frequency, permanent drift expansion and flattening tests, and regular dimensional checks. A thorough and detailed inspection of the raw material is essential. The effort involved in documenting the machine parameters with commercially available equipment and software, which has proven to be valid in individual cases in the event of quality deviations, must be estimated in every single case. The ultimate goal is always the same: to achieve your customer’s needs and pass the endurance tests with your tubes!
About the author:
Juergen Stindt is a graduate mechanical engineer, aged 60, with 27+ years of experience in leading tube manufacturing companies in Europe and the USA, each in management positions. Over the last five years, he worked at the forefront of the tube industry as a project manager for companies in the field of ERW and DOM tubes—mainly aimed at manufacturing high-strength steel vehicle stabilizers. Among others, he was responsible for cooperating with machine manufacturers (particularly welding machine suppliers) from design to commissioning at the installation site. Since autumn 2021, he has been working as an independent consultant in tube production and for welding machine manufacturers.
