Spring 2009 (contents):

1. Austenite: Mother of all Microstructures

2. Feedback Control

3. Berlin Airlift - Welding Saves a City


Austenite: Mother of Microstructures


I am sure most of you recall the first gulf war as the mother of all wars and perhaps the present state of world economy could be defined as the mother of all recessions but I bet you probably never heard of the mother of all microstructures.  That credit goes to Austenite.
As molten steel solidifies, it inevitably becomes austenite at high temperature.  Austenite has carbon held in interstitial solid solution.  As austenite cools, it is no longer stable and has to transform to ferrite which has practically no solubility for carbon.  The ejected carbon forms cementite.  At eutectic composition, austenite breaks down into fine bands of ferrite and cementite; this phase is called pearlitewhich is as an equilibrium phase.  Mixture of ferrite and pearlite is common in carbon and low-alloy steels.
In many welding processes, the cooling rate is fast enough that the equilibrium phases are bypassed and a whole slew of non-equilibrium phases can form with unique properties; the most well known of them all is martensite.  Most high carbon steels such as those used in tool steels can form substantial amount of martensite on cooling.  Martensite is a hard and brittle phase that forms by a diffusionless transformation where the carbon atoms are trapped in place.  With proper post-weld heat treatment, martensite can be softened to reduce hardness while increasing toughness.  Other microstructures that can evolve from austenite are bainite and acicular ferrite.  With the right alloying elements, austenite itself can be forced to remain stable down to room temperature as in austentitic stainless steels including 304 and 316.  
Perhaps the single reason why steel has played such an important role as structural material of choice is the ability of austenite to give birth to such a wide variety of microstructures whose mechanical properties can be tailored to suit.  Definitely worthy of the moniker - Mother Of Microstructures.
Source:  Welding Metallurgy, Vol. I, Fundamentals, Publisher: AWS


Feedback Control - The Future of Welding
History of development of welding and joining processes can be categorized into three phases.  Processes that were based purely on supplying a source of heat such as brazing and soldering can be traced back thousands of years.  Once electricity was discovered and mankind was able to store and deploy electrons, a whole slew of processes cropped up including resistance welding, family of arc-welding processes, and electron beam welding; conversion of electrical signal into ultrasonic frequencies led to ultrasonic welding.  The most recent invention has been laser welding but that too was in the seventies.  So you might as well ask -What have they done for me lately?
Over the last three decades, most of the progress in welding equipment can be traced to improvement in control systems thanks to rapid progress in electronics and sensor technologies.  Controls can be divided into two types: inner loop andouter loop.  Inner loop is where the power supply has built-in systems to ensure it is functioning correctly.  For example, the power supply can ensure that it is supplying current (or voltage, laser power, or ultrasonic amplitude, etc.) within a very narrow tolerance limit irrespective of changes such as ambient temperature or incoming line voltage fluctuations.  However, no matter how well the inner loop is functioning, it is still not able to compensate for part-to-part variations (part alignment, contamination, change in laser focus, or tip distance) that affect process robustness.  Here we need the outer loop where sensors will pickup signals that indicate changes in the welding process due to external events.  The signals are then transferred to the inner loop where corresponding predetermined modifications are implemented in the energy delivery to effectively compensate for such variations.  Outer loop feedback controls have been refined quite well for resistance welding since the external signals picked up are relatively easy to measure including current, voltage, and displacement.  Such progress has been slow in coming for other processes such as arc welding and laser welding where picking up signals (without noise) is quite difficult.  The outer loop is where we are likely to see substantial progress going forward in the field of welding equipment.  For example, a fast response monitoring device can sense initiation of coupling of laser energy and signal the power supply to quickly reduce power to avoid expulsion.  Or an arc welding machine could be setup to sense an increase in weld pool temperature and use that information to turn down the welding energy accordingly.  Welding processes could definitely improve with some good feedback.  Seems like welding and humans have something in common after all.



 Berlin Airlift - Welding Saves A City
At the end of WWII, Germany was divided into east and west half and so was the capital city of Berlin.  Only problem was that Berlin was landlocked in the eastern half.  Once the soviets decided to make life miserable for the western allies, they blocked all land routes into Berlin.  That is when the allies decided to supply the city by air.  It was estimated that to keep 2 million people alive, a total of 3500 tons of supplies per day would be needed including coal, food, and other necessities.  Berlin had only two airports and needed a third one.  However, building an airport required heavy earth-moving equipment but none of the planes were big enough at airlift the machinery.  So the only solution possible was to cut the heavy earthmoving equipment and transport it in smaller pieces.  The cut parts were then welded together in Berlin.  The rebuilt machines were then used to build a brand new airport, Tegel Airfield, which incidentally is the main airport of Berlin today.  Nice to know that welding played an important role in saving the city.  Go Welding!!!