In cold worked steels, failures can be broadly nucleated by localized defects, such as seams, pipe and exogenous inclusions. The second are those which result from exceeding the strength of the material itself.
Being bullish on precision manufacturing is a logical conclusion from the strength of these just released economic indicators.
The PMPA Business Trends Report has a sentiment indicator for Sales for three months ahead. In January 2017, that indicator exploded, up 40% from December’s already optimistic number. (see the blue line on the chart below.)
But don’t take our word for it. Here are 7 economic indicators released last week that are at multi year highs:
Consumer Price Index (CPI) up 2.8% year over year in February, up from 0.9% in July and at a pace not seen since February 2012;
Producer Price Index (PPI) up 2.2% year over year, Highest year over year rate since May 2014;
Manufacturing Production up 0.5% in February. Manufacturing Output is up 1.2% over the last 12 months, fastest pace since April 2015;
Housing Market Index hit a 12 year high, its highest level since June 2005;
Housing starts rose 3.0% in February. Single Family housing starts jumped from 819,000 to 872,000 a pace unseen since October 2007;
Index of Consumer Sentiment from University of Michigan was close to January’s reading which was the best reading for this indicator in 13 years;
Retail sales were reported at 5.7 % just below January’s 6.0% year over year pace, highest since 2012.
The weight of the evidence is clear.
The weight of the evidence makes it difficult to be anything but bullish on our business prospects. What are YOU doing to take advantage of the opportunities that this strong economy is providing for YOUR precision machining shop?
Data courtesy Chad Moutray Ph.D., CBE, Chief Economist at National Association of Manufacturers Monday Economic Report
Decarburization on surface layers can affect heat treatment and hardness attained on parts. Decarburization also provides evidence of where in a process a defect or imperfection occurred.
Most defects in steel workpieces encountered in our precision machine shops are longitudinal in nature. While their presence alone is enough to concern us, for the purposes of corrective action, it becomes important to identify where in the process the longitudinal imperfection first occurred. Visual examination alone is not enough to confirm the source. Did it occur prior to rolling? During rolling? After rolling? Understanding decarburization and how it presents in a sample can help us to identify where and when in the process the imperfection first occurred.
The question that we want to answer as part of our investigation is usually “When in the process did the defect first occur?” Looking at decarburization and any subscale present can help us answer that question with authority. What is Decarburization?
“Decarburization is the loss of carbon from a surface layer of a carbon containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.”– Metals Handbook Desk Edition
The carbon and alloy steels that we machine contain carbon. In the photo above, the carbon is contained in the pearlite (darker) grains. The white grains are ferrite. In an etched sample, decarburization surrounding a defect is identified as a layer of ferrite with very little, or none of the darker pearlitic structure typically seen in the balance of the material. The black intrusion in the photo above is the mount material that has filled in the crevice of the seam defect. What is Subscale?
Subscale is a reaction product of Oxygen from the atmosphere with various alloying elements as a result of time at high temperatures. The presence or absence of the subscale is the indicator that helps us to pinpoint the origin of the defect. For a subscale to be present, the time at temperature must be sufficient for oxygen to diffuse and react with the material within the defect. According to Felice and Repp, 2250 degrees F and fifteen minutes is necessary to develop an identifiable subscale. Lower temperatures would require longer times. Typically rolling mill reheat cycles offer plenty of time to develop a subscale in a prior existing defect. However, for defects that are created during rolling, the limited time at temperature and the decreasing temperatures on cooling make formation of subscales unlikely. Reading Decarb and Subscale to Understand the Defect
Decarburization is time and temperature dependent. This means that its relative depth and severity are clues as to time at temperature, though interpretation requires experience and understanding of the differences in appearance from grade to grade based on Carbon content. Symmetrical Decarburization
If the decarburization is symmetrical this is an indication that the defect was present in billet or bloom prior to reheat and rolling. oxygen in the high temperature atmosphere of the reheat furnace depletes the carbon equally from both sides of the pre-existing defect. Asymmetrical Decarburization
Decarburization that is obviously asymmetrical indicates that the defect is mechanical in nature and was induced some time during the hot rolling process. Ferrite Fingers
Ferrite fingers are a surface quality problem that is associated with longitudinal bar defects. During reheat, a defect in the bllom or billet is exposed to high temperature atmosphere, forming decarburization and subscale around the defect. Rolling partially closes or “welds shut” the crack. However, a trail of of subscale is entrained in a formation of almost pure ferrite which has been depleted of pearlite, carbon and alloy by the reaction at elevated temperature. This trapped scale remains a potential oxygen source, driving further internal oxidation and decarburization if temperatures remain high. Continuous improvement requires taking root cause corrective action. Obviously identifying the root cause is critical. When we encounter longitudinal linear defects in our steel products, using a micro to characterize the nature of the decarburization and presence or absence of sub scale or ferrite fingers are important evidence as to when, where, and how in the process the defect originated.
“Stress cracks are defined as transverse or near transverse open crevices created when concentration of residual stresses exceed the local yield strength at the temperature of crack formation. These stresses can be mechanically induced or can be attributable to extreme temperature differences and /or phase transformations. They can originate at almost any point in the manufacture of the steel.”– AISI Manual Detection, Classification, and Elimination of Rod and Bar Surface Defects
Stress cracks are often found visually at locations that experience bending or straightening. They are also referred to as “Cross Cracks” or “Transverse Cracks.” Originally they were identified in mill billet and bloom products, prior to rolling. Micro examination can help determine crack origin by noting:
Presence of scale
Presence of subscale
Additional microstructural characteristics can reveal the thermal history of heating and cooling at the crack location.
Causes and Corrective Action
Excessive load during straightening can exceed the local yield strength of the material causing it to crack; reduce load applied by machine, or consider tempering or stress relieving material prior to straightening or further cold work.
Cooling too quickly can also induce stress cracks. Critical cooling rates are highly dependent on steel chemistry. Crack sensitive chemistries (Medium carbon and high carbon steels; also medium and high carbon steels with straight chromium or straight manganese additions.) These steels should be slowly cooled through transformation temperatures to minimize the occurrence.
Design faults such as
Heavy sections adjacent to light sections and sharp corners
Failure to fillet sharp corners
Use of fillets rather than tapers
Overloading the material during fabrication, processing, or application.
Detection of stress cracks is problematic as their transverseorientation makes them difficult to detect on equipment set up to detect longitudinal defects.
Final caveat: The term stress crack is arbitrarily defined based on industrial usage in the market. It does not necessarily imply anything about the specific metallurgical nature of the crack, I know that a number of people use the term “stress crack” to describe longitudinal cracks on steel bar products as well, which the AISI calls “Strain Cracks.”