Q What is the normal variation for tensile strength results, and how do you reduce the variation? For gray iron G3000, we have tensile results of 31 ksi to 34 ksi. Is this acceptable?

Tensile strength is one of the characteristics most commonly specified by purchasers of metal castings, as it gives a good indication that the part will function as designed.

The tensile test verifies the strength of the metal used to make a heat of castings. Generally, the process starts when a sample of metal is poured into a mold designed for producing tensile bar samples. Once the metal solidifies, the sample is machined to specific dimensions and pulled apart by a device that measures the force required to do so. Once the bar is pulled apart, the strength of the metal is calculated in terms of pounds per square inch.

Control Variables for Less Variation

There are numerous variables that affect the tensile test results of any cast iron sample, so a range of results from 31 to 34 ksi is acceptable.

Charge Material--Consistency begins with the charge material--the returns bin, scrap bin and alloy room. The quality of the scrap and returns used to make the melt are critical. A number of issues need to be checked:

* Are they clean and free of oil, cutting fluids and excess sand and rest?

* Are the returns of consistent chemistry?

* Do you really know the chemistry of your scrap?

* Is the scrap introducing unknown tramp elements that can affect mechanical properties of the metal?

The chemistry of the melt also should be checked on a regular basis. Tensile test results will be more consistent if the melt is controlled to a specific targeted chemistry for each element by making additions or dilutions when needed.

Test Samples--The next step of controlling test results are in the areas of melting, pouring, section size and cooling of the test bar samples. Once again consistency and process control in these areas is important.

* Melt and Pour Consistency. The melt cycle and super heating should be kept to a minimum. Test results will be more consistent if the test bar sample is poured from the same point in the heat concentration and from the same point from a given pour ladle. As a general rule, pour your sample from the middle of the heat concentration and from the middle of a pour ladle and avoid the first and last metal out of the furnace.

* Section size. The cooling and section size of the test bar sample also should be controlled. The specification for this can be determined by the metalcasting facility and may include input from the customer. The test bar sample section size should be close to the section size of the casting it is representing or should follow a standard specification such as ASTM A-48. The cooling of the casting can be simulated by letting the test bar mold go through shakeout with the castings, or the test samples could be allowed to cool to room temperature and then shaken out separately.

* Sample Preparation. Test bar sample preparation is the next step, and care should be exercised when cutting off the section of metal to be machined into the test bar and when machining the test bar. Using a band saw to cut the sample will avoid any microstructure changes that occur from the excessive heat generated from a cut-off wheel. Also, there should be sufficient machine stock to eliminate any surface effect from the mold-metal interface and the rapid solidification that occurs in that area. Machining of the sample also is very critical, with strict adherence to all specification dimensions being required for the least amount of test variation. Every test bar should be measured before being pulled to assure the cross sectional area is what it is believed to be.

Testing Equipment--The tensile testing machine needs regular calibration, good pulling head alignment and a qualified operator. The metalcasting facility may consider pouring a number of standard test bars. The bars should all be poured from the same ladle using all of the guidelines discussed above. These bars can then be pulled at regular intervals.

Test Bar Quality--Test bars that have been pulled should be examined for any internal defects that show up on the fracture surface. Internal defects could easily lead to lower tensile strength results. Results from test bars that fracture out of gauge length should be used only after careful consideration and examination.

AFS Technical Department, Schaumburg, Illinois 

Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice, c/o MODERN CASTING, at 847/824-7848 or [e-mail].