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  • Stainless steel 304 vs 316 / A2 vs A4

    What is the difference between 304 stainless steel and 316 stainless steel or in other words what is the difference between A2 and A4 stainless steel. 

    So let me begin. 

    Where does steel come from? 

    Steel comes from Iron ore found in the ground. Iron ore is a mix of iron and oxygen molecules. The iron ore is melted down through a process called smelting to remove the oxygen and turn it into Metallic Iron. 

    Raw Iron Ore

     



     Raw Iron Ore

     

    What do we do with the iron ore?

    Although the basic ingredients of an iron ore blast furnace has changed little over the last hundred years or so, the furnaces themselves have been modernized to improve efficiency and the purity of the iron and steel.

    The blast furnace has an improved and more efficient shape to suit automation, with the contents of iron ore, limestone and coke being carefully measured, conveyed and loaded into the furnace from the top through specially designed gas-tight valves. High temperature air at over 1000C (which is can be heated using the exhaust gasses from the furnace much like a boiler economiser) is blasted into the bottom of the furnace through several tuyeres.

    The coke is used as a fuel to produce the high temperatures required by the process of smelting the ore in a blast furnace. It produces CO which adds heat, as well as acting as a reducer removing the oxygen content from the ore.

    Limestone is used as a flux to separate the gangue from the molten iron ore, the accumulated slag and the molten iron being tapped from two tap-holes at the bottom of the furnace. The slag goes to a disposal area and the molten iron is directed into molds known as pigs where it solidifies to pig iron and is transferred to the next stage of processing.

    Turning the Iron Ore into steel

    The basic raw material for steel manufacture is either the hot metal from the blast furnace, steel scrap or a mixture of both. The proportions of material used vary according to the process and the type of steel required. Steel can be described in general terms as iron with most of the carbon removed, to make it tougher. There are many forms (grades) of steel, each with its own specific chemical composition and properties to meet the needs of the many different applications. Two major steel making processes are used today in the UK.

    Basic oxygen steelmaking and Electric Arc Furnace 

    Basic oxygen steelmaking 

    Using the hot metal from the furnace above and scrap steel are the principal materials in BOS.  A water-cooled oxygen lance is lowered into the converter and high-purity oxygen is blown on to the metal at very high pressure.

     

    Basic oxygen furnace

     

    The oxygen combines with carbon and other unwanted elements, eliminating them from the molten charge. These oxidation reactions produce heat, and the temperature of the metal is controlled by the quantity of added scrap.Modern furnaces, or ‘converters’ will take a charge of up to 350 tonnes and convert it into steel in around 15 minutes. 

    The carbon exits the converter as a gas, carbon monoxide, which can, after cleaning, be collected for re-use as a fuel. During the ‘blow’, lime is added as a flux to help carry off the othbasic oxygen steelmaking process diagramer oxidized impurities as a floating layer of slag . The quantities of scrap, hot metal and lime and other fluxes are calculated to ensure the correct steel temperature and composition. In many plants, the refining process is assisted by the injection of gases, including argon, nitrogen and carbon dioxide, through the base of the furnace. After the steel has been refined and samples taken to check temperature and composition, the converter is tilted and the steel is tapped into a ladle. Typically, the carbon content of the steel at the end of refining is about 0.04%. During tapping, alloy additions can be made to adjust the final composition of the steel.

    When all the steel has been tapped, the converter is turned upside down and the residual slag is tipped into a waiting slag ladle for removal to a slag cooling pond, from where it is further processed to reclaim any material which can be returned to the process.

     

    Electric Arc Furnace 

    The EAF mostly uses cold scrap metal only. This process was first used for making the best steel available to be used for tools and springs as there is more control over the composition. Today, however, it is also employed in making more widely used steels, including alloy and stainless grades as well as some special carbon and low-alloy steels. 

     

     

     

     

    With what looks like a round bath with a removable roof, through which three graphite electrodes can be raised or lowered. At the start of the process, the electrodes are withdrawn and the roof swung clear. The steel scrap is then charged into electric arc furnacethe furnace from a large steel basket lowered from an overhead travelling crane. When charging is complete, the roof is swung back into position and the electrodes lowered into the furnace. A powerful electric current is passed through the charge, an arc is created, and the heat generated melts the scrap. Lime and fluorspar are added as fluxes and oxygen is blown into the melt. As a result, impurities in the metal combine to form a liquid slag

    Samples of the steel are taken and analysed to theirElectric arc furnace check composition and, when the correct composition and temperature have been achieved, the furnace is tapped rapidly into a ladle. Final adjustments to precise customer specification can be made by adding alloys during tapping.

     

    How is stainless steel is made?

    Stainless steel is made up from some of the basic elements found in the earth: iron ore, chromium, silicon, nickel, carbon, nitrogen and manganese. Varying the amounts of these substances creates the different grades of stainless steel. 

    Stainless steel can be made up from scrap steel.

    Stainless steel is produced in an electric arc furnace where carbon electrodes contact recycled stainless scrap and various alloys of chromium (and nickel, molybdenum etc. depending on the stainless type). A current is passed through the electrode and the temperature increases to a point where the scrap and alloys melt. The molten material from the electric furnace is then transferred into an AOD (Argon Oxygen Decarbonization) vessel, where the carbon levels are reduced (remember stainless has a much lower carbon level than mild steel) and the final alloy additions are made to make the exact chemistry. The mix is then melted and casted either into ingots or continually cast into a slab or billet form. Then the material is hot rolled or forged into its final form. Some material receives cold rolling to further reduce the thickness as in sheets or drawn into smaller diameters as in rods and wire.

    Most stainless steels receive a final annealing (a heat treatment that softens the structure) and pickling (an acid wash that removes furnace scale from annealing and helps promote the passive surface film that naturally occurs).

    The fact that stainless steel has a great resistance to corrosion means that using stainless will result in a very long life compared to mild steel. Structures made from stainless steel will last many times the normal life (well over 100 years in most cases). So, while stainless steel is probably more expensive to buy in the beginning -- because it lasts a long time, it is usually cheaper in the long run because there is little or no maintenance and repair costs. AND, once the useful life is over, stainless steel is 100% RECYCLABLE. Scrap stainless steel is recharged into the electric furnaces for re-melting back into stainless steel. Stainless steel is a true "full life cycle" material.

    So what are the differences between the main types of stainless steel. 

    What is the difference between 304 and 316 stainless steel? 

    There are many grades of stainless steel but in this blog we shall look at the two main types. Each of these types of stainless have their pros and cons, both are manufactured to meet differing environmental and physical demands. So let take a look at the main points of 304 stainless steel and 316 stainless steel. 

    1.    Composition of SS
    This is the first main difference between 304 grade stainless steel and 316 grade stainless steel. Chromium and alloy content and crystalline structure determines the steel classification. Austenitic steel – also known as 300 series steel – contains 18% chromium and 8% nickel. This type of steel is incredibly common; about 70% of stainless steel products are composed of austenitic steel. 304 grade stainless steel is considered to be austenitic, while 316 grade stainless steel is considered to be superaustenitic – meaning it contains a higher content of molybdenum, nitrogen and nickel, making it more resistant to corrosion and stress.

    2.    Resistance to Corrosion
    304 stainless steel also known as A2 stainless steel is resistant to corrosion however 316 stainless steel also known as A4 stainless steel provides a higher degree of corrosion resistance because it contains molybdenum in its composition. Therefore making 316 A4 stainless steel a better suited steel for resistance to extreme environmental conditions like exposure to chemical solutions, brine solutions, de-icing salts and of course salt water. 304 stainless steel or A2 as some like to call it will tarnish after a while but will resist corrosion for a time 

    3.    Applications
    Stainless steel type 304 is used frequently in a variety of contexts. It is often found in household products and appliances, including pots and pans, coffee urns, stoves, refrigerators and sinks. It is also commonly used in dairy equipment and the brewing and food producing industries. 304 grade stainless steel is also used frequently for nuts, bolts, screws and other fasteners.

    Stainless steel type 316 is more commonly used in severe environments and for industrial processes that require higher levels of resistance to corrosion than 304 grade stainless steel can offer. This type of metal is often used in chemical processing equipment, to create fasteners and hardware used for industrial purposes, and in surgical implants. It is also frequently used in marine environments because of its higher resistance to tarnish and corrosion. It is also widely used in marine environments were its resistance to salt water is crucial. All fittings at J Clarke Marine are A4 316 stainless steel unless other wise stated. We try to source the best products available on the market that will last for as long as possible. 

    4.    Cost
    Typically, 316 grade stainless steel costs slightly more than 304 grade steel because of its enhanced chemical and production properties. The extra cost can be justified, however, because hardware made of type 316 stainless steel is extra resistant to tarnish and corrosion, enabling it to last a very long time.


    5.  What chemicals are used in Stainless steel? 

     

    Chemicals Type 304 Type 316
    Carbon 0.08% max 0.08% max
    Manganese 2.00 % max 2.00% max
    Phosphorus 0.045% max 0.045% max
    Sulfur 0.030% max 0.030% max
    Silicon 1.00% max 1.00% max
    Chromium 18.00-20.00%  16.00-18.00
    Nickel 8.00-10.50% 10.00-14.00
    Molybdenum 2.00-3.00%

     

    6.  Is stainless steel magnetic??

    It is often asked whether 316 A4 stainless steel or 304 A2 stainless steel is magnetic. Lets take a look

    ................to be continued

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  • Comments on this post (14 comments)

    • Elisa says...

      Thank you for this very informative article! Precisely the details I was looking for regarding SS bolts for a job I am working on.
      I look forward to poking around your blog to see what else I can find of interest!

      On September 27, 2017

    • Brenton Jones says...

      generally a very good article. I wanted to know about the differences between 304 and 316 stainless steel, also known as A2 and A4 apparently. Got this at the end of the article. The first part describing STEEL being made of Iron was a bit irrelevant as it is a different subject. 304, 314, 316 Stainless steel is really an Alloy containing no iron what so ever. It should be called a chromium alloy, not a steel I would think.

      On August 25, 2017

    • andrew scott says...

      Simple, clear, logical, concise, extremely helpful. thank you.

      On June 11, 2017

    • Edmund Butler says...

      Excellent article. Many thanks!

      On May 25, 2017

    • David Bourne says...

      Terrific article – exactly what I was looking for. Came up as the first hit on an intertubes search. Which,these days is somewhat amazing…. Thanks again.

      On May 17, 2017

    • Mike Bailey says...

      Brilliant, exactly what I was looking for, very well written and easy to understand

      On March 03, 2017

    • Neil says...

      Wow, actually got the information I was looking for first hit!
      Great article and many thanks for taking the time to do it.

      On October 04, 2016

    • Stephen says...

      Thanks for a great article. One question I have is which is the stronger between 304 & 316? I’ve read that 316 tends to be more brittle, and therefore more prone to cracks/chips? I’m asking specifically in respect to grinding gears for a kitchen juicer.
      Any advice here?
      Thanks.

      On August 05, 2016

    • Alan Cardwell says...

      This is a very interesting and informative article. I am installing a new shower but, for various reasons, I can’t use the screws which came with it – ie screws to fix the alluminium profiles to the tiled walls. I hope to get longer stainless steel screws to do the job. Could you please say whether I should look for A2 or A4 grade screws and also whether they could cause corrosion of the alluminium? Many thanks.

      On May 29, 2016

    • Hugh Pickering says...

      Extremely useful and informitive article. Thanks very much

      On May 27, 2016

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