do not respond to hardening. Their properties, however, can be influenced by cold working. They are magnetic at room temperature. The microstructure in the annealed condition consists of ferrite grains in which fine carbides are embedded. Ferritic steels used for machining contain a large amount of manganese sulfides to facilitate free cutting.respond to heat treatment. Martensite is formed through rapid cooling. Properties can then be optimized by subsequent tempering treatment. The alloys are magnetic. Depending on the thermal treatment, the microstructure can range from a pure martensitic structure to fine-tempered martensite. Different alloys and various dimensions of semi-finished products require complex heat treatment temperatures and times.Fig. 6: Martensitic chrome steel, electrolytically polished and etched with A2. Bright field.In some corrosion resistant steel welds, a certain amount of delta ferrite is needed to improve hot-crack resistance. However, delta ferrite is usually an unwanted phase, because the long annealing times of steel with a high chromium content can change the delta ferrite into the hard and brittle iron-chromium intermetallic sigma phase. Heating up to 1,050 °C and subsequent quenching removes the sigma phase and with it the embrittlement.do not respond to thermal treatment. Instead, rapid cooling results in the production of their softest condition. In this state, they are non-magnetic and their properties are influenced by cold working. The microstructure of these steels consists of austenite grains, which may exhibit twinning.Fig. 7: Austenitic steel with twins and segeragtions. Colour etched with Lichtenegger and Bloech. DIC.Fig. 8: Deltaferrite in an austenitic steel weld (small dark strings) and larger deltaferrite lines in weld part (blue-grey); electrolytically etched with 40 % aqueous sodium hydroxide solution. Bright fieldExposing these steels to elevated temperatures in the region of 600-700 °C can result in the formation of complex carbides within the austenite grains. This leads to an impoverishment of chromium in the austenite solid solution, which increases the susceptibility to intergranular corrosion or oxidation.Fig. 9: Austenitc steel tube with twins and deformation from cold working; etched with 10 % oxalic acid, DICBy reducing the carbon to below 0.015 % and adding small amounts of titanium, niobium or tantalum, the risk of intergranular corrosion is reduced, as these elements form carbides in preference to the chrome. Delta ferrite can appear due to critical heat treatment conditions in martensitic steels or cold working of austenitic steels.Fig. 10: Strings of deltaferrite in austenitic steel matrix, electrolytically etched with sodium hydroxide in water (20 %)(duplex) consist of ferrite and austenite. Electrolytic etching in a 20…40 % caustic soda solution reveals the structure, and the correct percentage of each phase can be estimated. These steels are ductile and are specifically used in the food, paper and petroleum industries.Fig. 11: Forged duplex steel showing blue ferrite, light to dark brown austenite. Double electrolytical etching; first 10 % oxalic acid in water and second etch 20 % sodium hydroxide in water; DIC
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Bend line– The straight line on the surface of the sheet, on either side of the bend, that defines he end of the level flange and the start of the bend.
Bend radius – The distance from the bend axis to the inside surface of the material, between the bend lines.
Bend angle – The angle of the bend, measured between the bent flange and its original position, or as the included angle between perpendicular lines drawn from the bend lines.Sometimes specified as the inside bend radius. The outside bend radius is equal to the inside bend radius plus the sheet thickness.
Neutral axis – The location in the sheet that is neither stretched nor compressed, and therefore remains at a constant length.
K-factor – The location of the neutral axis in the material, calculated as the ratio of the distance of the neutral axis T, to the material thickness t. The K-factor is dependent upon several factors (material, bending operation, bend angle, etc.) and is greater than 0.25, but cannot exceed 0.50. K factor = T/t
Bend allowance – The length of the neutral axis between the bend lines or the arc length of the bend. The bend allowance added to the flange lengths is equal to the total flat length.
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The K-factor is the ratio between the the neutral axis to the thickness of the material.
Importance of the K-factor in sheet metal design
The K-factor is used to calculate flat patterns because it is related to how much material is stretched during bending. Therefore it is important to have the value correct in CAD software. The value of the K-factor should range between 0 – 0,5. To be more exact the K-factor can be calculated taking the average of 3 samples from bent parts and plugging the measurements of bend allowance, bend angle, material thickness and inner radius into the following formula:
Some basic K-factor values are shown here. Use these as a guideline.
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