Steel is the most important construction material for cranes because of its load-carrying capacity, its favourable processing behaviour and its high efficiency in mobile crane engineering. The metal’s properties can be adapted to suit the respective technical demands. High-strength steels used in mobile crane structures allow light weight designs that reduce capital costs and also operating costs. And it means that larger capacity mobile and crawler cranes are possible.
In mobile crane engineering the dead weight of the construction has a critical influence on the working load, and with that, on the efficiency of the mobile crane. Reducing the dead weight without losing load-bearing capacity (i.e. the strength and component safety of the construction) is a primary requirement.
Thyssen Krupp Stahl AG, for example, caters for these demands with its quenched and tempered structural steels available with minimum yield strengths up to 1,100MPa. Fig.1 illustrates the weight-saving possibilities of using high-strength steels compared to standard grade S355.
In spite of their high strength, these steels have outstanding toughness, cold forming quality advantages and good weldability, thanks to modern steel making metallurgy and rolling and heat treatment technology. Narrow thickness tolerances can also be achieved and there is high resistance to brittle fracture.
Evaluation of modern high strength steel plate components, using safety concepts based on fracture mechanics, has shown that these steels exhibit a toughness level that allows safe operation of welded structures even under critical service conditions. The use of high-strength steels allows the implementation of new technical solutions in mobile crane engineering and has contributed to the creation of a new generation of cranes.
The evolution of high-strength structural steels with yield strengths above 550MPa began at Thyssen Krupp about 30 years ago. The desired minimum yield strengths could only be achieved through special rolling, and/or heat treatment techniques like thermo mechanical rolling and/or water quenching, coupled with adequate steel compositions.
Steel making
Thyssen Krupp steel is produced according to the Thyssen blowing-metallurgy (TBM) procedure. The liquid steel is stirred when gas is blown through the converter bottom which achieves a better mixing of metal and slag. TBM allows a lower content of phosphorus and sulphur as well as a higher degree of purity.
Ladle metallurgy is similarly important as it helps the converter process, allows a precise impression of the targeted chemical composition as well as a setting of the sulphur content to extremely low values. That helps purity and results in reducing brittle fracture and an outstanding isotropy of the toughness and deformation properties. In future the near net shape casting will gain importance during the production of flat products.
Next to a balanced chemical composition of the steel and modern metallurgy, the use of modern rolling techniques and heat treatment methods is vital. In this case normalised and/or normalised rolled steels, thermo-mechanically rolled and/or accelerated cooled steels as well as quenched and tempered steels with different chemical compositions are used. Fig.2 shows a general survey of important production methods of high-strength steels, typical microstructure, areas of application and the resultant yield strength.
For the highest demands of strength and toughness in mobile crane engineering, water quenched steel grades of the type N-A-XTRA and XABO® are produced. XABO® 1100 is a peak in the evolution of high-strength structural steels. Heat treatment begins with heating up to temperatures above Ac3. Rapid cooling follows, using pressurised water to quench the heavy steel plates.
Besides chemical composition the tempering treatment after quenching is the most important control factor for the mechanical properties. In future it is conceivable that the process can be simplified through direct quenching from the rolling-heat. During this process an improvement of the hardenability results which can be used also to further decrease the alloying content and improve weldability. Thyssen Krupp uses the special advantages of the heavy plate production via hot strip rolling during the production of water quenched steels up to thicknesses of 12mm and widths up to 2,000mm. The modern production method gives good thickness tolerances and minimises additional costs which create new possibilities for optimising mobile crane engineering construction and further reducing dead weight and processing costs.
Alloying and properties
Well proven steels for mobile crane construction are low-alloyed steels with a carbon content of less than 0.2%. They contain additions of chromium, molybdenum, nickel and, where appropriate, a micro-alloying with niobium and vanadium according to the required minimum yield strength and thickness of plate. The most important alloying elements, the carbon equivalent CET and the characteristic mechanical properties of the most common mobile crane structural steels are identified in Fig.3.
Forming and welding
To a large extent mobile crane engineering employs cold formed heavy plates. Compared with general structural steels that have a lower yield strength, greater forces are needed for cold working of water-quenched structural steels. For cold formability the results of the technological bending test are important. The lower the relation of bending diameter to thickness the better the formability. Fig.4 documents the minimum bending radii for high-strength structural steels for incipient crack-free bending. Compared with previous steel grades the modern steels can have very low minimum bending radii and at larger plate thicknesses.
High-strength structural steels are well suited to conventional welding procedures and in mobile crane engineering the preference is for shielded gas welding. Important aspects during welding are cold crack safety and the application-orientated mechanical properties of the welded joints. For both aspects there are many influential factors. At Thyssen Krupp systematic basic investigations were carried out in this field. As a result, the highest demands on the load-bearing capacity of welded constructions of high-strength structural steels can be achieved.
High-strength steels show a high cold crack safety through their low carbon equivalent content and the use of high-quality filler metals with a low hydrogen content that are usual in the shielded gas welding process. Considerations represented in EN 1011 to avoid cold cracks (CET-concept) show that with thicker plates of high-strength steels preheating for the hydrogen effusion and delayed weld cooling is necessary.
Welding conditions also influence the mechanical properties of the welded joints of high-strength structural steels. The cooling-time t8/5 is important. It is proportional to the heat input. The cooling-time t8/5 should, with modern structural steels, be between 10s and 25s in order to guarantee optimum properties in the welded joint.
From the combination of the cooling times (heat input) and the pre-heating temperature determined with the CET concept as optimal for cold cracking safety, working diagrams can be drawn up. The working diagram for S690 QL (N-A-XTRA 70) is shown in Fig.6 as an example. From such diagrams application-orientated welding conditions for all high-strength steels can be determined according to material thickness.
Brittle fracture behaviour
When designing welded structures knowledge of the failure and deformation behaviour is important for a safe construction. To verify the component safety of welded steel structures, particularly against sudden failure through brittle fracture, quantitative safety analyses are increasingly applied on the basis of fracture-mechanical calculations. The safety analysis is carried out by comparison of the toughness of the component, resulting from the demand situation carried out, with the measured fracture-mechanical toughness of the material in the welded joint.
An example of the corresponding safety analyses carried out to be able to judge the possibility of secure operation of welded constructions of high strength steel is shown in Fig.7, a comparison of toughness demands and toughness levels of steels up to 1,100MPa minimum yield strength according to Eurocode 3. It becomes clear that demands on the toughness increase with strength. For the considered steels it is thought, however, in all yield strength ranges, that the toughness reserve is always above the demands. The real fracture toughness of the steels is therefore always above the fracture-mechanical component toughness. Usefully for the toughness reserve is the possibility to go to lower thicknesses in the course of the design with incremental high-strength steels. All high-strength steels also show a toughness level which is more than sufficient for the safe operation of welded structures even in the lowest service temperatures.
Stability behaviour/fatigue strength
In the case of static loads the yield strength of high-strength structural steels can be used if there is only tensile stress in the component. On the other hand, in the case of static pressure loads, the stability criteria for dimensioning are to consider buckling and bulging. In comparison with conventional steels it is shown that high-strength structural steels are usually charged at low slenderness ratios more highly.
Mobile crane structures are loaded not only statically but also dynamically. It is important in this case to consider that the resistance of high-strength steels against dynamic loads does not increase in comparison with conventional steels to the same extent as the yield strength. This long known fact is shown in Fig.8, in which the Woehler curves of the conventional steel S355, and the high-strength steel S960QL, are opposed to each other. In spite of more than twice the yield strength the S960QL shows a difference of about 50% in the fatigue-strength-for-finite-life field. The fatigue limit of the high-strength steels is only up to 33% higher than at S355. In addition, it is demonstrated that welded joints of high-strength structural steels are only slightly superior to conventional steels under unfavourable conditions with regard to the number of stress cycles, tensile ratio (relationship of under to upper stress) and notch-case, (Fig.8). Nevertheless high-strength steels in welded constructions can always be usefully applied if the admissible fatigue-strength of the high-strength steel exceeds the acceptable static strength of the conventional material. This is the case with high mean stress, low load cycle number, a favourable form of the load collective and small notching.
To maximise the advantages of high-strength steels in the case of fatigue loads, it appears that as few welds as possible should be placed in stressed areas of the construction. Attempts have been made for some time to improve the fatigue-strength of welded joints through weld treatment procedures. Shot peening, grinding and the TIG dressing of the joint are some examples.
The improvement effect is based in the case of the grinding and TIG dressing on a decrease of the notching of the weld toe notch in the weld metal/base material transition. During shot peening a favourable residual stress-state is built up in the surface layer. These processes can significantly increase the fatigue strength both in the fatigue-strength-for-finite-life field and also in the fatigue-limit field. Even the fatigue-strength of the base material can be improved (Fig.9).
Processing and manufacturing
Mobile crane manufacturer Liebherr-Werk Ehingen, for example, orders all its high-strength structural steels with a 3.1B-material report to DIN 10204 and a detailed specification. In this the thickness tolerances and flatness are limited below the usual range. The steel is cut using autogenous flame cutting, plasma or laser. During cold deformation folding-radii are defined according to the rolling-direction. After cutting the surface is cleaned to Jet degree of purity Sa 2 1/2. Preparation of the weld edge is done by autogenous flame-cuts, edge bevelling machines or flexible belt grinders (Fig.10).
To avoid cold cracking, the area to be welded is preheated. This leads to a delay of the cooling rate after welding, supports hydrogen effusion and reduces the residual strength level. The necessary preheating temperature depends on the carbon equivalent, the hydrogen content and the thickness of plate. The default and/or calculated minimum preheating temperature must be accurately maintained and continuously monitored during welding. Liebherr Ehingen uses the ‘MAGM’ (shielded gas welding with mixed gas) procedure. When joining larger thicknesses of plate the multi-layer technique is used (Fig.12).
To a large extent the aim is to adapt the mechanical properties of the welded joints to those of the basic material. Correspondingly, alloyed filler metals are used for that. For S960QL the root can be welded with a low-strength, ductile G4Si1 and the filling and covering-layers with a welding addition SG 960. The same strength values are reached by the alloying with the base material and the applied cooling-time concept t8/5 in the welded joint as at the base material. In this case the t8/5 is the time that the welding layer and their heat affected zone need to cool down from 800°C to 500°C. Cooling-time is influenced basically by the preheating temperature, the heat input and the weld geometry.
Through careful welding, with preheating and cooling, the formation of hydrogen induced cold cracks – which develop perpendicular to the welding direction – can be avoided. Welds are examined using the usual non-destructive tests for cracks. As well as dependence on the test equipment, and the determined test range, weld quality is defined with the statically added weld factor. Quality of the welded construction is guaranteed by a great number of design and manufacturing guidelines for the high-strength structural steels.
