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High strength seamless tubes and hollow sections for cranes and machine building applications PRODUCTION AND PROPERTIES – REPRINT FROM „STAHLBAU”, ISSUE 9, 2015

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High strength seamless tubes and hollow sections for cranes and machine building applications

PRODUCTION AND PROPERTIES – REPRINT FROM „STAHLBAU”, ISSUE 9, 2015

Cover_Sonderdruck_A4_23.05.16.indd 3 24.05.2016 08:44:08

Page 2: High strength seamless tubes and hollow sections for ... · PDF fileT. MüllerB. Straetmans High strength seamless tubes and steel hollow sections for cranes and machine building applications
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Topics

DOI: 10.1002/stab.201510310

© Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Stahlbau 84 (2015), Heft 9, S. 650–654

Thomas MüllerBoris Straetmans

High strength seamless tubes and hollow sections for cranes and machine building applications Production and Properties

1 Introduction

Over decades, materials with higher yield-strengths have been developed for the production of seamless hot rolled tubes and hollow sections. To-day, quenched and tempered high-strength tubes and hollow sections are available with yield strengths of up to 960 MPa. These are used, for example, in crane construction, hy-draulic cylinders, and in other frame constructions in machine engineering which are subject to static or dynamic loads, for instance in agriculture ma-chinery (Fig. 1).

2 Requirements

The development of high-strength ma-terials has been motivated by the need to handle higher static, dynamic, and lifting loads, and larger dimensions, as well as by a desire to extend service life, and by other factors that influ-ence the production. These require-

High strength tubes and hollow sections for cranes and machine building applications are used with increasing utilization of the designs to cover continuously growing requirements. For many applications limiting construction weight is a necessity. Either regulatory inputs, like axle load limitation of mobile cranes, or other application-specific requirements such as load capacity of crawler cranes are covered. Additionally high strength steels al-low wall thickness reductions and thereby give opportunity to re-duce the effort of welding. For design and execution beside yield strength and static properties mostly ductility, impact properties and preferable easy processing through uniform properties of the used steel grade are key interests. This article will provide an overview of requirements, manufacturing and properties of high strength seamless tubes and hollow sections.

Hochfeste nahtlose Rohre und Stahlbauhohlprofile für Krane und den Maschinenbau – Herstellung und Eigenschaften. Hochfeste

Fig. 1. Applications for improved and high-strength steel tubes and structural hol-low sections from left to right lattice crawler crane Liebherr LR13000 (Liebherr), hy-draulic cylinder (iStock.com/pic4you), agricultural machinery (iStock.com/pic4you)Bild 1. Anwendungsbereiche für höher- und hochfeste Stahlrohre und Stahlbau-hohlprofile, v. l. n. r. Gittermast raupenkran LR13000 (Fa. Liebherr); Hydraulikzy-linder (iStock.com/pic4you), Landmaschinen (iStock.com/pic4you)

Rohre und eckige Stahlbauhohlprofile für Krane sowie für den Maschinenbau werden eingesetzt, um bei zunehmender Auslas­tung den stetig wachsenden Anforderungen an die Konstruktion gerecht zu werden. In vielen Anwendungen besteht die Notwen­digkeit, Konstruktionsmassen zu begrenzen. Damit werden ent­weder regulatorische Vorgaben, wie beispielsweise Achslastbe­schränkungen bei Mobilkranen, oder andere anwendungsspezi­fische Erfordernisse erfüllt, wie die Steigerung der Hakenlast bei Gittermastkranen. Zudem bietet sich die Möglichkeit, Wanddi­cken zu reduzieren und damit den Verarbeitungsaufwand beim Schweißen zu reduzieren. Für Bemessung und Ausführung sind neben der Streckgrenze und den statischen Eigenschaften meist die Duktilität, die Kerbschlagzähigkeit sowie eine möglichst einfa­che Verarbeitung durch gleichmäßige Eigenschaften der einge­setzten Werkstoffe von zentralem Interesse. Dieser Beitrag bietet einen Überblick über Anforderungen, Herstellung und Eigen­schaften hochfester nahtloser Rohre und Stahlbauhohlprofile.

ments derived from engineering standards, guidelines, and laws led to a primary focus on developing high-strength materials. Several research projects were promoted to meet struc-tural requirements in terms of mate-

rial properties and processing tech-niques ([1], [2], [3]).

Increased automation changes the boundary conditions in manufac-turing facilities. The consequence is a need for higher precision in cuttings,

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After piercing, all metalworking processes continue with the first es-sential forming, and stretching respec-tively, according to the rolling proce-dure relevant for the specific dimen-sion. As the temperature of the hollow block drops during the forming pro-cess, the steel must be reheated to roll-ing temperature in a reheating furnace before it can be rolled to the final di-mensions in a sizing or stretch-reduc-ing mill. The roll stands of these mills facilitate the production of round tubes and square/rectangular sections. Elliptical dimensions are also feasi-ble. The last steps of production are straightening, non-destructive tests, cutting, marking, and if necessary, bundling [4].

4 Material properties of high-strength seamless tubes and hollow sections

The development of fine-grained steels has already begun in the 1950ies. To-day it includes higher-strength mate-rials up to approx. 500 MPa yield strength, and high-strength heat treated materials up to 960 MPa. Pro-gress in metallurgical engineering, such as the development of materials with low phosphorus and sulphur contents, and substantially improved purity paved the way for these steels with high yield strength and increased ductility [5].

Higher-strength fine-grained steel with a minimum yield strength of up

compensation of material tolerances, and the application of sophisticated separating processes which have little or almost no influence on the material. This is the basis to successfully process high-strength steel with homogeneous properties, even at the welding seams.

3 Production of seamless tubes and hollow sections

The basic material for seamless tube production is usually continuous cast-ing. Regardless of the subsequent seamless tube rolling process, the ba-sic material is cropped to the required

length. In a rotary hearth furnace the steel is then heated up to rolling tem-perature, before a cross rolling pro-cess forms it into a hollow block. This ingenious invention was made by the Mannesmann brothers at the end of the 19th century. To this day it is the basis for almost all seamless tube roll-ing processes. In order to produce tubes with large dimensions and heavy weights per meter, some rolling processes require the use of ingot cast-ing. After heating, the ingot goes through a piercing press where it is pre-pierced and transformed into a hollow block (Fig. 2).

Fig. 2. Scheme of processes for seamless tube productionBild 2. Schematische Übersicht der Nahtloswarmwalzverfahren

Table 1. Essential mechanical properties of some fine grain steel grades (extract) out of EN 10210-1 [6] and FineXcell®-material data sheets in comparison to a standard grade S355J2HTabelle 1. Wesentliche mechanische Eigenschaften einiger Feinkornstahlsorten (Auszug) der EN 10210-1 [6] und FineXcell®-Werkstoffdatenblätter im Vergleich zu einer Standardgüte S355J2H

Strength category

Grade Heat treatment Tensile test Notch impact test

Reh, and Rp0,2min

in MPa

Rm

in MPa

Amin.in %

Test tem-perature

in °C

KVminin J

normal- strength

S355J2H rolled

355 470 to 630 22

–20 27

S355NH normalized –20 40

S355NLH –50 27

higher- strength

S460NH460 540 to 720 17

–20 40

S460NLH –50 27

high-strength S690QL hardened and tempered 690 770 to 960 16

–40 45

S690QL1 –60 40

S770QL 770 820 to 1 000 15 –40 45

S890Q890 960 to 1 110 14

–40 45

S890QL1 –60 30

S960QL 960 980 to 1 150 10 –40 27

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Sonderdruck aus: Stahlbau 84 (2015), Heft 9

5 Processing and application

All standard welding methods can be applied to weld high-strength tubes and hollow sections. To achieve simi-lar properties in the welding seams, as well as in the base material, the me-chanical and technological properties of the filler metals should match those of the base material. High-strength fine-grained steels require particular attention to the properties in the heat-affected zone (HAZ), as well as to the cold cracking behaviour. Aside from the choice of material and weld metal this property change is also set by a process parameter: the cool-down time t8/5.

With increasing material strength it becomes more important to meet the defined processing specifications. Optimised heat conduction whilst ob-serving the specified t8/5 time is there-fore of particular importance for the processing of high-strength materials ([7], [8]). Trained and experienced welding operators can easily meet the t8/5 time by adhering to the pre-heat-ing and interpass temperature, and by

to 500 MPa, and high requirements on toughness, as for example 40 J at –20 °C or 27 J at –50 °C, are normal-ized. But they can also be made in a normalizing rolling process with re-gulated temperature control. High-strength fine-grained steels with mini-mum yield strengths of 500 MPa to 960 MPa are hardened and tempered in a separate process, after hot rolling. For this purpose they are heated up to  austenitizing temperature, water quenched, and then tempered at de-fined temperatures and retention times (Fig. 3).

The heat treatment process (Fig. 3) thus guarantees a yield strength that usually starts at mini-mum 690 MPa. This marks a signifi-cant difference to higher-strength materials. The guaranteed minimum ductility is subject to steel grades, and can be up to 16 %. The notch impact strength is guaranteed between 27 J to 50 J, and temperatures from –40 °C, –50 °C or –60 °C.

After the heat treatment, the mi-cro structure and the chemical com-position of high-strength tubes and structural steel sections lead to funda-mentally different mechanical prop-erties, compared to higher-strength steels and simple mild steels (e.g. S355J2H according to EN 10210 [6]) in as-rolled state (Fig. 4).

Low-alloy fine-grained steels have a carbon content of below 0.20 %. Whilst the properties of high-er-strength fine-grained steels are mainly achieved through normalizing heat treatment in  combination with grain refining V microalloying. To achieve quenched and tempered high-strength fine-grained steels it is neces-sary to use solid solution elements, like Cr, Mo, Ni and W, in order to achieve basis strengths. These are ad-ditionally combined with V, Nb and

Ti as grain-refining elements. Prefera-bly, V is used in combination with N in a defined stoichiometry to form ho-mogeneously distributed carbon-ni-tride precipitates, which inhibit the grain growth and increase the recrys-tallisation stopping temperature over the final rolling temperature. This pro-cedure induces an increased strength through deformation solidification [5]. It is generally accepted that grain- boundary strengthening according to the Hall–Petch relation has a benefi-cial effect on ductility, and increases the yield strength (Fig. 5).

Fig. 3. Heat treatment processBild 3. Vergütungsprozess

Fig. 4. Microstructure stateBild 4. Gefügezustände

Fig. 5. Schematic about influence of grain size to impact behaviorBild 5. Schematische Darstellung des Einflusses der Korngröße auf die Kerb-schlagzähigkeit

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considering sheet thickness, welding velocity, and welding beads (Fig. 6).

At the same wall thickness, high-strength fine-grained steels allow a greater load on the construction, compared to S355J2H steels; at a re-duced wall thickness they have the same effect, provided that admissible strain is not exceeded. Under the given set of boundary conditions the characteristics of the material, includ-ing the connections, guarantee good material behaviour in terms of distor-tion and high ductility.

The influence of material fatigue is of growing importance in machine engineering, but also in other fields, such as bridge engineering. The num-ber of cycles-to-failure that need to be withstood to bear stress without dam-ages, range from a few thousand (Low Cycle Fatigue) to a fatigue strength of 2 · 106. The stress range of bearable cycles-to-failure (fatigue strength) is primarily influenced by stress in-creases that occur at the notches. Welded joints constitute geometric and metallurgic notches. Hence, they are of special interest in the fatigue load analysis of high-strength materi-als. Various post-treatments on the welding seam (high frequency im-pact treatment), such as hammering, grinding etc. help to increase fatigue strength and to achieve a higher notch impact class.

This treatment also inhibits crack-growth of already formed cracks. Failure induced by breakage occur thus only at a high number of cycles-to-failure.

Particularly in regards to short-term strength, components made of high-strength steels benefit from higher nominal yield strengths. This

6 Summary

Higher and high-strength tubes and hollow sections combine high yield strengths with excellent strain behav-iour and good notch impact proper-ties. They are therefore suitable for applications that have to meet high technical demands and standards. The continuous homogeneous proper-ties of hot-formed materials, tight ge-ometric tolerances, and the weld-ing-related processing have leveraged this class of materials over recent years. The applied research on these applications has promoted their pop-ularity and made significant contribu-tions to enhance the processing qual-ity. In this respect, high-strength mate-rials usefully complement existing processes, to meet future challenges in construction and manufacturing technology.

Bibliography

[1] Ummenhofer, T., Herion, S., Hrabow-ski, J., Feldmann, M., Eichler, B., Bu-cak, Ö., Lorenz, J., Boos, B., Eiwan, C., Stötzel, J.: Bemessung von er müdungs-be an spruch ten Bauteilen aus hoch und ultrahochfesten Feinkornbaustählen im Kran und Anlagenbau. Forschungs-bericht P778. Forschungsvereinigung Stahlanwendung e.V., Düsseldorf: Ver-lag und Vertriebsgesellschaft mbH 2013.

[2] Ummenhofer, T., Veselcic, M., Diet-rich, R., Nussbaumer, A., Zamiri, F.: Optimaler Einsatz von Hohlprofilen und Gussknoten im Brückenbau aus Stahl S355 bis S690. Düsseldorf: Verlag und Vertriesgesellschaft mbH 2014.

[3] Puthli, R., Herion, S., Bergers, J., Sed-lacek, G., Müller, C., Stötzel, J., Höh-

phenomenon allows for wider stress ranges. Furthermore, these can bear higher stress levels (medium stress). An updated version of EN 13001-3-1 [11] based on new research results has been drafted over recent years, taking into account existing rules and stand-ards (e.g. EN 1993-1-9 [9] and FKM- guideline [10]).

Aside from applying high-strength tubes, the use of structural steel sections of up to 960 MPa yield strength has lately been established. As with the production of tubes, the seamless hot-forming process benefits from manufacturing conditions, i.e. the homogeneous micro structure without metallurgic notches (welding seam). Additionally, low residual stress in the entire cross section re-duces the risk of distortion. This char-acteristic also provides excellent workability, especially at the profile corners.

Small corner radii allow for the positioning of bore-holes and welding seams close to the corners. This cre-ates larger connection faces. Recent developments facilitate the optimisa-tion of the already small corner radii of hot-formed hollow sections. Conse-quently, this allows for the design of corner radii with the maximum size of a single nominal wall thickness.

In comparison to tubes, struc-tural steel sections feature simpler cutting- patterns and clear connection geometry with sections of straight welding joints. The advantage over cold-formed hollow sections is a clearly reduced welding seam volume (Fig. 7), and a more efficient construc-tion.

Fig. 6. Fabrication of steelconstruction hollow section nodeBild 6. Herstellung eines Stahlbau-hohlprofilknotens

Fig. 7. Connection using different corner radiiBild 7. Anschluss bei unterschiedlichen Kantenradien

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ler, S., Bucak, Ö., Lorenz, J.: Beurteilung des Ermüdungsverhaltens von Kran konstruktionen bei Einsatz hoch- und ultrahochfester Stähle. Forschungs-bericht P 512. Forschungsvereinigung Stahlanwendung e.V., Düsseldorf: Ver-lag und Vertriebsgesellschaft mbH 2006.

[4] Kümmerling, R.: Das Schrägwalzen von Rohren. Stahl und Eisen 109 (1993), H. 9, S. 503–511.

[5] Bruns, C., Müller, T., Liedke, M., Scheller, W.: Schweißen hochfester Nahtlos-Rohre für den Kranbau. DVS Berichte Band 267 , S. 440–446, 2010.

[6] EN 10210: Warmgefertigte Hohlpro-file für den Stahlbau aus unlegierten Baustählen und aus Feinkornbau stäh-

len – Teil 1: Technische Lieferbedin-gungen, Teil 2: Grenzabmaße, Maße und statische Werte, CEN, 2006.

[7] Bruns, C., Müller, T., Liedke, M., Schel-ler, W.: Schweißen im Kranbau – Naht-eigenschaften hochfester Rohre. DVS Berichte Band 275, S. 399–405, 2011.

[8] SEW 088 Schweißgeeignete Feinkorn-baustähle. Richtlinien für die Verarbei-tung, besonders für das Schmelzschwei-ßen. Düsseldorf: Verlag Stahleisen mbH 1993.

[9] DIN EN 1993-1-9: Eurocode 3: Be-mes sung und Konstruktion von Stahl-bauten – Teil 1-9: Ermüdung. Deutsche Fassung EN 1993-1-9:2005 + AC:2009, 2010-12.

[10] FKM-Richtlinie – Rechnerischer Fes-tigkeitsnachweis für Maschinenbau-teile. VDMA-Verlag, 2012.

[11] DIN EN 13001-3-1: Krane – Kons-truktion allgemein – Teil 3-1: Grenzzu-stände und Sicherheitsnachweis von Stahltragwerken, CEN, 2013-12.

Authors of this article: Dr.-Ing. Thomas Müller, [email protected], Dipl.-Ing. Boris Straetmans, [email protected], Vallourec Deutschland GmbH, Theodorstraße 109,40472 Düsseldorf

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