For investment-grade companies, the average yield on bonds was 2.77 percent as of Wednesday, according to Bloomberg Barclays index data. In late November, that figure was above 4.3 percent. For a company selling $1 billion of debt, that amounts to $15.3 million of annual interest savings, before taxes. Junk-bond yields have dropped too, with notes rated in the BB tier, the uppermost high-yield levels, paying a near-record-low 4.07 percent.

It's not clear how long that will last—on Thursday, U.S. Treasury yields surged, with the 10-year note jumping as much as 0.12 percentage point, to 1.59 percent. But for now the bond sales are intense enough to make up for a year that had previously been lackluster. Investment-grade issuance is now down just about 2 percent from the same point last year. In June, the gap was closer to 13 percent.

The recent spate of issuance is the latest surge in corporate debt sales, as companies have ramped up their borrowings to buy back shares and invest in new projects. Investment-grade debt outstanding totaled $5.8 trillion on Wednesday, more than double the level a decade ago.

The prior record for investment-grade bond issuance was $66 billion in the week of Sept. 9, 2013, when Verizon Communications Inc. sold $49 billion of bonds in eight parts, the biggest corporate debt offering ever. Companies now are by and large refinancing maturing debt, instead of funding big new capital projects.

The underwriting fees that the sales are generating are one of the few positives for bank profits that are expected to get hit by falling rates. The refinancing can also translate into greater trading revenues, said Bloomberg Intelligence analyst Arnold Kakuda.

It's a stunning turnaround from late last year, when Scott Minerd, Guggenheim Partners' global chief investment officer, said a selloff in General Electric Co. debt signaled that "the slide and collapse in investment grade credit has begun." While the investment-grade market last year generated losses of 2.5 percent, this year it's up 14.2 percent including both interest and price gains, making it one of the best-performing assets in fixed income.

In the leveraged-loan market, 17 deals totaling more than $16 billion have launched this week, making it the busiest week since October. Investment-grade and high-yield bankers are telling clients that the good times may not last.

"If someone has near-term financing needs, they should be looking to take advantage of this window," said Jenny Lee, co-head of leveraged loan and high-yield capital markets at JPMorgan Chase & Co. "Things potentially could shut down or get more difficult as we head toward the back half of this year."

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