- Hardness or Hardness Test
- Heat Affected Zone
- Hoop Stress
- Hot Work
- Hydrogen Blistering
- Hydrogen Sulfide
- Impact Wrench
- Intergranular Fracture
- Intermittent Cracking
- Keeper Link
- Knoop Microhardness
- Keyence Digital Microscope (KDM)
- Lack of Fusion (LOF)
- Lack of Penetration (LOP)
- Low Magnification SEM
Hardness or Hardness Test: The resistance of metal to penetration or scratching is called hardness. In a hardness test, an indenter of known shape and size is pushed or forced into a metal surface with a known pre-established pushing load. The size of the penetration or resistance to penetration is measured. This resistance to penetration is reported as a number dependent on the type of indicator and the load causing the indention. One of the first and simplest hardness tests was the Scratch Scale. In the Scratch Scale of Hardness, the materials that were the most scratch resistant were above and harder than those with lesser resistance to scratching.
Heat Affected Zone (HAZ): A zone or region of metal adjacent to the weld bead but in the base metal. During welding the temperature in the heat affected zone will quickly increase and then again decrease as a result of the production of the weld bead. The effect of heat on the base metal microstructure and local properties, such as strength and hardness is more dramatic near the weld and diminishes at distances into the base metal away from the weld bead.
Holiday: A hole or void in a coating (paint) or pipe wrapping.
Hoop Stress: A circumferential stress, which, in a pipe or pressure vessel would tend to make the pipe diameter or circumference increase. As fluid that has filled the pipe is pressurized, the hoop stress causes the diameter or circumference to increase.
Hot Work: Welding and cutting work where extreme heat and high temperatures is expected and permitted.
Hydrogen Blistering: The agglomeration of hydrogen atoms which have usually differed into the metal from an external source. Blisters, delamination and internal separations occur when the hydrogen atoms unite to form hydrogen gas.
Hydrogen Sulfide (H2S): A chemical compound consisting of two atoms of hydrogen (H) combined with one atom of sulfur (S). Hydrogen sulfide is poisonous and has a strong odor resembling that of rotten eggs. Hydrogen sulfide reacts and corrodes numerous metals and alloys.
I.D.: Short notation for inside diameter, meaning the inside surface or diameter of the pipe.
Impact Wrench: An air or electric powered wrench in which repeated blows from an internal hammer are used to generate torque in a fastener/joint. Impact wrenches CANNOT be used as a substitute for a torque wrench.
Insoluble: Does not integrate with or dissolve. Not able to dissolve in a solvent.
Intergranular Fracture: A fracture mode where failure occurs by separation along the metal grain boundaries and/or failure in a near grain boundary layer rather than through the metal grains. Intergranular fracture is sometimes described as looking at having a rock candy like fracture appearance. Intergranular fracture is evidence of zero or near zero ductility and is generally not a desirable fracture mode. A scanning electron micrograph (SEM) showing the intergranular fracture at a magnification of 1000x is shown in the below photograph.
Intermittent Cracking: Cracking that is stop-go-stop-go, sometimes referred to as step-wise cracking, progressive cracking, or intermittent cracking.
Interstitial: The small gap, interval or interspace between objects or parts. In the figure below, several interstitial areas are marked in red and denoted with arrows.
Intrados: The inner or interior curve or surface of an arch or curved object.
Keeper Link: An oval or link used to keep a pelican hook beak in place in order to contain or trap the wire rope or chain link within the pelican hook.
Keyence Digital Microscope (KDM): A computer assisted microscope which allows the operator to take photographs on adjacent focal planes. The KDM computer then analyzes, captures, gathers all in focus pixels from the various focal planes, and combines them into in focus, three dimensional color images.
Knoop Microhardness: Knoop microhardness is determined on a polished metal surface. The Knoop indenter is an elongated pyramidal diamond with a long and short axis.
Ksi: Thousands of pounds per square inch. A shortened expression for psi. That is 35,000 psi is more easily read as 35 ksi.
Lack of Fusion (LOF): A soldering flaw or defect that occurs when the solder joint does not fuse or bond with one or more pieces of metal that are being joined by the soldering process. The most common cause of lack of fusion is a poor welding technique; another cause is the use of a very wide weld joint. If the welding arc is directed down the center of the joint, the molten weld metal will only flow and be solidified and cast against the sidewalls of the base plate without melting them.
Lack of Penetration (LOP): A welding flaw or defect in which the surface of the metal to be joined has not melted. Melting of the metal surface permits the weld bead filler metal, that metal added by the welding rod/wire, to melt and mix with the surface of the metal to be joined. A LOP flaw or defect will appear many times as an almost straight line where the melted bead merely lays on the cold metal surface without any joining of the two metal surfaces. This is usually caused by the use of too low a welding current and can be eliminated by simply increasing the amperage. Other cause can be the use of too slow a travel speed, an incorrect torch angle or too narrow of a gap between pieces.
Low Magnification SEM: A scanning electron microscope (SEM) can be used to take good, three dimensional, black and white photographs of small objects or samples. An example that most people are familiar with are the numerous photographs published in the news and other media outlets of ticks, fleas, bedding mites, and other small bugs. At high magnifications, these monsters can appear fearsome but in reality the picture could be of a bug the size of a grain of sand.
An example using Low Magnification SEM at a magnification of 20x is shown below next to a digital photograph of a piece of debris left in a nasal cavity after surgery.
Photograph of debris left in a patient's nasal cavity.
Although the digital photograph shown above is very revealing, a much better three-dimensional view (although only in black and white) is shown below.
Debris left in nasal cavity (mag 20x).
This SEM photograph is at a magnification of 20x and shows the topography (shape) of the subject debris in great detail.
Another use of the SEM for low magnification photography was an investigation involving fuel clip failures from a truck fuel injector. An overall macroscopic photograph of a failed injector clip is shown below.
Failed truck engine fuel injector clip.
An SEM view of the failed end of the fuel nozzle clip is shown in the SEM scanning electron micrograph shown below.
Failed end of a fuel injector clip (mag 35x).
Even at a magnification of only 35x in the photo above, it is evident that the fracture made is intergranular cracking.