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A couple standard tests for weld quality are a tension test, where a section of weldment is pulled apart; and the bend test, where a section of weldment is bent on the weld. If it fails on the weld, the weld (and if it is in a class, the weldor) is failed and does not pass go. These guys have pictures of both fixtures on site.

All of this is a long-winded way of saying a proper weld is stronger than the base metal. Obviously, here, we're dealing with a defective weld, so you're quite correct to say "When the piece hardens, you have something that, while not as strong a a solid piece, has enough strength to resist whatever pressures might be pushing on it." However, in the general case, the weld will be stronger than the base metal.

Loved "The Physics of NASCAR".

Thank you (*blush*) for the comment about Physics of NASCAR.

A welding seminar I attended for welding rollcages in racecars emphasized that you choose a weld material that is weaker than the materials you are joining. Am I remembering wrong about that?

Great Post. I'm always interested in hearing about how things are made - or why they fail.

Regarding the cost of repairing infrastructure - it doesn't cost as much as you might think from first glance. It creates a lot of jobs, which means that a good chunk of the money comes back to the government in the form of payroll taxes. If the materials are purchased from American companies, then quite a bit of that money comes back in taxes, too. It puts people to work, which means that it's probably one of the most effective ways to get money into the economy. I haven't been able to understand why politicians don't see that we have an infrastructure problem and an unemployment problem, and come up with a solution to both.

Actually you are wrong on one point.. The weld, if done properly, is stronger than the base metal, and hence the joint is stronger. This is determined by the type of welding rod used. For instance a 6010p5 rod has 60k lbs tensile strength, and a 7018 low hydrogen rod has 70k.

Also, you would be stunned if you knew the condition of pipelines in the ground. Many suffer from SCC (stress corrosion cracking), and external and internal corrosion. This pipe was running at a paltry 375 psi. Triple that is the norm in rural areas.

Further, just because a pipe is rated at 375 psi does NOT mean it will burst at 380 psi. 375 is the MAOP or maximum allowable operating pressure. When pipelines are hydro tested after construction, test pressures are typically 1.5 times the desired working pressure. This line was likely tested to 563 psi, which is still far from 100% of SMYC or the actual pressure that will cause a rupture.

As mentioned above, I bet they find microscopic SCC on the surface of the pipe. This does not look like a weld failure, as evidenced by the ripping longitudinally, and not following a spiral or straight seam, or butt weld.

All in all this is way too complex to cover in a short blog piece.

You should write something on the fascinating technology involved in testing pipelines that are in service and under pressure, called smart-pigging.

I asked over here and, while the responses somewhat support my position, they don't link to any documentation. I did find AWS has a set of standards which talk in depth on the issue, but at a couple hundred bucks a pop, I'll pass.

Yes, I agree Paul, way too complex for a short blog.

Paul is correct! The weld, if done properly, is stronger than the pipe. Unfortunately pipeline welding codes only require something like 60% weld penetration.... I don't know the exact number but it is in the right area.

There are also many other factors that compromise the weld quality like letting the weld cool down between passes, wind, electrode moisture and many others. For example if a weld is cooled down to quickly then the material is like to have coarse grain structure in the heat affected zone. That results in material failure like in this failed weld test coupon. I have a page about welding certifications that gives a good rundown about weld quality and testing.

Another problem I am seeing is cheap pipe and metals. I personally weld for a living and can't believe how companies are cutting corners these days. They are using everything except American products and the quality is terrible! Cheap electrodes, bad metals and now they are cutting labor cost by more the half. The new trend is if you don't speak English and don't question anything then we will hire you. Real scary!

If companies really wanted weld quality they would TIG weld the pipes (superior quality welds), use quality materials (American or not) and hire skilled labor!

In the end I don't think companies will replace infrastructure unless it is the worst case scenario. It just comes down to paying for a few deaths cost less then it would to fix the problem. Just think of the BP oil spill. Nobody remembers that anymore and they are still making money!

Dave: I talked to a weldor about this, and they said the issue here with safety gear is that you weld with lower-grade filler to reduce the heat affected zone. They don't (contrary to what I expected) put airframes or car cages into an over to stress relieve them, so they need as small a HAZ as possible.

Diandra was given some specialized information on an exceptional process and generalized from it; probably the reason it was emphasized in the forum she was in was due to the exceptional nature of that type of welding.

Diandra, if you're still reading along, thanks for writing here, there, and everywhere.

Ry: I certainly am reading along! The beauty of blogging in a forum like this is that I learn as much from my readers as I do writing. I write enough "authoritative" stuff in my real life. I appreciate that the folks attending this particular cocktail party are knowledgeable and willing to share without needing to put each other down. Thank you!

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    Physics Cocktails

    • Heavy G
      The perfect pick-me-up when gravity gets you down.
      2 oz Tequila
      2 oz Triple sec
      2 oz Rose's sweetened lime juice
      7-Up or Sprite
      Mix tequila, triple sec and lime juice in a shaker and pour into a margarita glass. (Salted rim and ice are optional.) Top off with 7-Up/Sprite and let the weight of the world lift off your shoulders.
    • Listening to the Drums of Feynman
      The perfect nightcap after a long day struggling with QED equations.
      1 oz dark rum
      1/2 oz light rum
      1 oz Tia Maria
      2 oz light cream
      Crushed ice
      1/8 tsp ground nutmeg
      In a shaker half-filled with ice, combine the dark and light rum, Tia Maria, and cream. Shake well. Strain into an old fashioned glass almost filled with crushed ice. Dust with the nutmeg, and serve. Bongos optional.
    • Combustible Edison
      Electrify your friends with amazing pyrotechnics!
      2 oz brandy
      1 oz Campari
      1 oz fresh lemon juice
      Combine Campari and lemon juice in shaker filled with cracked ice. Shake and strain into chilled cocktail glass. Heat brandy in chafing dish, then ignite and pour into glass. Cocktail Go BOOM! Plus, Fire = Pretty!
    • Hiroshima Bomber
      Dr. Strangelove's drink of choice.
      3/4 Triple sec
      1/4 oz Bailey's Irish Cream
      2-3 drops Grenadine
      Fill shot glass 3/4 with Triple Sec. Layer Bailey's on top. Drop Grenadine in center of shot; it should billow up like a mushroom cloud. Remember to "duck and cover."
    • Mad Scientist
      Any mad scientist will tell you that flames make drinking more fun. What good is science if no one gets hurt?
      1 oz Midori melon liqueur
      1-1/2 oz sour mix
      1 splash soda water
      151 proof rum
      Mix melon liqueur, sour mix and soda water with ice in shaker. Shake and strain into martini glass. Top with rum and ignite. Try to take over the world.
    • Laser Beam
      Warning: may result in amplified stimulated emission.
      1 oz Southern Comfort
      1/2 oz Amaretto
      1/2 oz sloe gin
      1/2 oz vodka
      1/2 oz Triple sec
      7 oz orange juice
      Combine all liquor in a full glass of ice. Shake well. Garnish with orange and cherry. Serve to attractive target of choice.
    • Quantum Theory
      Guaranteed to collapse your wave function:
      3/4 oz Rum
      1/2 oz Strega
      1/4 oz Grand Marnier
      2 oz Pineapple juice
      Fill with Sweet and sour
      Pour rum, strega and Grand Marnier into a collins glass. Add pineapple and fill with sweet and sour. Sip until all the day's super-positioned states disappear.
    • The Black Hole
      So called because after one of these, you have already passed the event horizon of inebriation.
      1 oz. Kahlua
      1 oz. vodka
      .5 oz. Cointreau or Triple Sec
      .5 oz. dark rum
      .5 oz. Amaretto
      Pour into an old-fashioned glass over (scant) ice. Stir gently. Watch time slow.