Mechanical Engineering - Applied Metallurgy

Project instead of Exam Part C

Projects

i POLITECNICO DI MILANO School of Industrial and Information Engineering 095838 – Applied Metallurgy Material selection: vessel of a fire extinguisher ii Summary The case study...................................................................................................................... 1 1.1. Analysis of a fire extinguisher ................................................................................. 1 1.2. Product requirements .............................................................................................. 2 1.2.1. Failure mode safety ......................................................................................... 2 1.2.2. Corrosion resistance ........................................................................................ 3 1.2.3. Operating range ............................................................................................... 3 1.2.4. Weld quality ..................................................................................................... 3 2.Materials and technological cycle ................................................................................... 4 2.1. Material selection .................................................................................................... 4 2.1.1. C40 .................................................................................................................. 4 2.1.2. X5CrNi18-10 .................................................................................................... 4 2.1.3. X2CrNiMoN22-5-3 ........................................................................................... 5 2.1.4. Al 6061-T6 ....................................................................................................... 5 2.2. Technological cycle ................................................................................................. 6 2.2.1. Manufacture of steel fire extinguisher .............................................................. 6 2.2.2. Manufacture of aluminium fire extinguisher ...................................................... 7 3.QFD analysis ................................................................................................................. 8 3.1. Description of the QFD analysis .............................................................................. 8 3.2. Conclusion ............................................................................................................ 10 4.Appendix ......................................................................................................................... i 4.1. Bibliography ............................................................................................................. i 1 The case study A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user (i.e., no escape route, smoke, explosion hazard, etc.), or otherwise requires the expertise of a fire brigade. Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire. Fire extinguishers manufactured with non-cylindrical pressure vessels also exist but are less common. They are kept handy at places, namely fire points, in buildings, factories, public places or transportation [1]. 1.1. Analysis of a fire extinguisher The main parts of a fire extinguisher are five: one or two tanks, a valve assembly, a hose (or a nozzle), an extinguishing agent and a propellant. •The tanks are typically made of metal; they contain the extinguishing agent and the propellant. The tank is linked to the valve assembly through a sleeve. •The valve regulates the extinguishing agent flow. It is composed of a body, a handle to lift the fire extinguisher, a locking pin, a release lever and a dip tube. In stored pressure fire extinguisher, we can also find a pressure gauge, or a pressure indicator. The body of the valve is made of stamped brass, molten aluminium or highly resistant technical resins. •The hose allows to direct the flow of the extinguishing agent. It is a flexible pipe, it can only be found on fire extinguishers that are heavier than 3 Kg. •The extinguishing agent is a substance that limits the spreading of the fire and allows for its extinction when it comes into contact with it. 2 •The propellant is a gas whose function is to expel the extinguishing agent from the fire extinguisher. [2] In our project, we will focus on the pressure vessel (the tank) and we will choose the material that better fits in order to carry out its purpose. 1.2. Product requirements 1.2.1. Failure mode safety In order to avoid the fast fracture of the component, it is analysed the case of study for material specification for a pressurized tank. It is assumed: ������ �=�������∙������ ������ where ������ � is the pressure at which there would be the plastic deformation of the component, ������ ������ is the operating pressure (which is taken around 70 bar [3]) and ������� is a safety coefficient. For a correct design of a pressurized tank, it must be: �=������ � � 2 ������ ������ and from the theory of fast fracture it must be: ������=������ ������ √� ������ where ������ is the stress intensity factor and ������ is the length of a pre-existent crack inside the component. It will be assumed ������=� in order to guarantee that the component will get damaged with plastic deformation mode and not with fast fracture mode also in case of a crack that has length equal to the thickness of the component. So: ������=������ ������ √� ������ � � 2 ������ ������ it will be the stress intensity factor achieved in plastic deformation condition by the component. In order to have a parameter that represents the failure mode safety for each material, it is chosen a sort of safety coefficient between ������ ������= √�� ������ (critical stress intensity factor) and the stress intensity factor achieved in plastic deformation condition by the component. This parameter is ������ ������ ������ that, considering fixed operative conditions and geometry, is proportional to: ������=������ ������ √������ ������ 3 So, changing only the material, ������ is an index of fast fracture resistance: the bigger ������, the more we are in safety conditions. 1.2.2. Corrosion resistance In order to avoid the corrosion of the vessel, it will be considered the corrosion process in the following analysis. The corrosion problem is related to: •Passivation of material •Electrochemical potential of material •% C in material •% Cr in material Passivation of material means the capability of material to form and to reconstruct a protective layer capable of stopping the corrosion process. Higher is the passivation capability, higher is the corrosion resistance of certain material. The electrochemical material influences corrosion problem: higher (in modulus) is the electrochemical potential of material, higher is the corrosion potentiality for a certain material. Higher is the percentage of carbon in a material, more important is the corrosion problem in a material. Higher is the percentage of chromium in a material, higher is the corrosion resistance of the material. 1.2.3. Operating range Analysing the operative temperature range of the fire extinguisher, problems related with low temperatures and cycling changes of temperature of the vessel will be considered. There will be an analysis on DBTT temperature of different material to avoid the change of behaviour of material from ductile to brittle and an analysis on tendency of certain material to stand cycling change of temperature so to resist the fatigue phenomenon associated with cycling temperatures loading. 1.2.4. Weld quality To study the welded part resistance, it will be a discussion about weld quality of components made by certain material. The weld quality depends on the HAZ (heat affected zone) of certain material and on percentage of carbon in the material. With the increase of the HAZ, the weld quality decreases and materials with high percentage of carbon are not easily weldable. 4 2.Materials and technological cycle In this section the most used materials in the production of the fire extinguishers’ pressure vessels in commerce are listed [4] [5]. The properties of each material are analysed, in order to highlight the advantages and the risks related to the material choice. Eventually, a brief description of the technological cycle is carried out. 2.1. Material selection 2.1.1. C40 The most economical material used in this application in the plain carbon steel C40. Having no relevant content of other chemical species, such as chromium, it is not able to resist to corrosion, so it is necessary an anti-corrosion painting treatment on the surface. The absence of nickel does not counteract the attitude of this steel to have a DBTT, thus reducing the operative range of the fire extinguisher. This material, with respect to stainless steels, can be treated thermally in different ways, in function of the mechanical performances required; being a medium carbon steels, it still has good weldability. Chemical composition Mechanical properties C (%) Si (%) Mn (%) P (%) S(%) ������ � [MPa] ������ ������� [MPa] Kc [MPa- √m]� [kg/m3 ] 0.37- 0.44