Fracture toughness properties of duplex stainless steels

Good toughness properties in base and weld material enable the use of duplex stainless steels (DSS) in critical applications. DSS offer high strength compared to common austenitic stainless steels. The high strength can be utilized to reduce the wall thickness and accordingly accomplish reduction of cost, welding time and transportation weight, contributing to ecological and energy savings. Although DSS have been used successfully in many applications the last decades, the full utilisation in pressure vessels has been restricted due to conservative design rules. The consequences of failure in a pressure vessel are often very severe and it is accordingly important to verify a high ductility and fracture toughness. In this study fracture toughness data has been generated that has been used to analyse the brittle failure model in the European pressure vessel code EN 13445. The evaluation of the results has been made successfully by the master curve analysis, previously applied to ferritic steels. The master curve analysis includes calculation of a reference temperature, which can be correlated to an impact toughness transition temperature. A correlation between fracture and impact toughness results is necessary for a practically applicable design code. The heat distribution and austenite reformation have been modelled to verify satisfactory toughness properties in the heat affected zone. A similar model was used to evaluate the nucleation and diffusional growth of sigma phase during isothermal heat treatment or continuous cooling. For future stainless steel development, the availability of satisfactory correlations between composition, microstructure and mechanical properties are essential to optimize alloy design. Stainless steel data has been analysed to find approximate relations between mechanical properties and the chemical composition, grain size, ferrite content, product thickness and solution hardening size misfit parameter. The solution hardening effect was successfully predicted by the Labusch-Nabarro relation and multiple regression analyses were used to evaluate hardening equations for stainless steel.QC 2010092

Understanding the Fatigue Notch Sensitivity of High-Strength Steels through Fracture Toughness

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2.Validerad;2023;Nivå 2;2023-08-10 (joosat);Licens fulltext: CC BY License</p

Increasing fatigue performance in AHSS thick sheet by surface treatments

Advanced High Strength Steels (AHSS) have been widely applied in the automotive industry as an affordable solution for car lightweighting, mainly in parts subjected to crash requirements. Heavy duty vehicle (HDV) can also benefit from the expertise learned in cars, but parts must be designed considering fatigue resistance, especially on trimmed areas, and stiffness. Mechanical surface treatments, as blasting or shot peening, help increasing fatigue life of AHSS in trimmed areas and will allow weight reduction in HDV through gauge downsizing. The expected decrease in stiffness through thickness reduction can be improved by design changes. However, scarce information about the effect of mechanical surface treatments on AHSS are available. Thus, the aim of this work is to evaluate the increment in fatigue life of two different steel grades (350 MPa, and 500MPa of yield strength) in thick sheet by means of mechanical surface treatment – sandblasting. High Cycle Fatigue [HCF] tests were conducted at alternating load [R=-1]. Residual stresses were measured by an X-ray tensometry prior fatigue tests. Also the surface roughness [Rz] and form is measured using an optical non-contact 3D microscope. On the other hand, the fracture surfaces of the test specimens were observed via scanning electron microscope (SEM) in order to determine the crack initiation points. The evaluation of fatigue life in terms of SN curves is also discussed, analysing how the sandblasting process modifies the surface roughness and introduce compressive residual stresses on the external layer of the material. Both phenomena enhance the fatigue strength of the evaluated steel grades

Increasing fatigue performance in AHSS thick sheet by surface treatments

Advanced High Strength Steels (AHSS) have been widely applied in the automotive industry as an affordable solution for car lightweighting, mainly in parts subjected to crash requirements. Heavy duty vehicle (HDV) can also benefit from the expertise learned in cars, but parts must be designed considering fatigue resistance, especially on trimmed areas, and stiffness. Mechanical surface treatments, as blasting or shot peening, help increasing fatigue life of AHSS in trimmed areas and will allow weight reduction in HDV through gauge downsizing. The expected decrease in stiffness through thickness reduction can be improved by design changes. However, scarce information about the effect of mechanical surface treatments on AHSS are available. Thus, the aim of this work is to evaluate the increment in fatigue life of two different steel grades (350 MPa, and 500MPa of yield strength) in thick sheet by means of mechanical surface treatment – sandblasting. High Cycle Fatigue [HCF] tests were conducted at alternating load [R=-1]. Residual stresses were measured by an X-ray tensometry prior fatigue tests. Also the surface roughness [Rz] and form is measured using an optical non-contact 3D microscope. On the other hand, the fracture surfaces of the test specimens were observed via scanning electron microscope (SEM) in order to determine the crack initiation points. The evaluation of fatigue life in terms of SN curves is also discussed, analysing how the sandblasting process modifies the surface roughness and introduce compressive residual stresses on the external layer of the material. Both phenomena enhance the fatigue strength of the evaluated steel grades

Warm Forming of Hot Rolled High Strength Steels with Enhanced Fatigue Resistance as a Lightweight Solution for Heavy Duty Vehicles

Most solutions for a lightweight design are based on the implementation of AHSS, Al alloys or CFRP. However, not all these strategies are susceptible to be applied to truck chassis parts. These components require high fatigue resistance and thick sheets. Additionally, they are usually trimmed and punched, which is known to affect fatigue resistance. This work addresses the lightweight con-struction of truck parts through the warm forming of steel grades tailored to show high formability and fatigue behaviour. The fatigue limit was evaluated for different edge conditions (polished and punched) and rationalized using the cracking resistance described by fracture toughness. The con-sideration of both mechanical properties, fatigue, and fracture toughness, gives an innovative and useful tool to develop and select materials for fatigue dimensioned parts.Funder: Research Fund for Coal and Steel RFCS (800649);ISBN för värdpublikation: 978-3-95735-150-0RFCS programme - Warmlight projec

Warm Forming of Hot Rolled High Strength Steels with Enhanced Fatigue Resistance as a Lightweight Solution for Heavy Duty Vehicles

Most solutions for a lightweight design are based on the implementation of AHSS, Al alloys or CFRP. However, not all these strategies are susceptible to be applied to truck chassis parts. These components require high fatigue resistance and thick sheets. Additionally, they are usually trimmed and punched, which is known to affect fatigue resistance. This work addresses the lightweight con-struction of truck parts through the warm forming of steel grades tailored to show high formability and fatigue behaviour. The fatigue limit was evaluated for different edge conditions (polished and punched) and rationalized using the cracking resistance described by fracture toughness. The con-sideration of both mechanical properties, fatigue, and fracture toughness, gives an innovative and useful tool to develop and select materials for fatigue dimensioned parts.Funder: Research Fund for Coal and Steel RFCS (800649);ISBN för värdpublikation: 978-3-95735-150-0RFCS programme - Warmlight projec